(GH-3136) Introduce CheckBoxHelper and new Windows 10 CheckBox Style

- A CheckBoxHelper was introduced to allow different Glyphs and Brushes for different Styles
- Introduced a new Windows 10 CheckBox Style that switches the colors for a better visibility, especially with a dark theme
	- Use Win10 check glyph Xaml path data from "Segoe MDL2 Assets"
	- Set Win10 BorderThickness to 2

src/MahApps.Metro.Samples/MahApps.Metro.Demo/ExampleViews/ButtonsExample.xaml

@@ -350,11 +350,13 @@
             <CheckBox Margin="{StaticResource ControlMargin}" Content="_Enabled" />
             <CheckBox Margin="{StaticResource ControlMargin}"
                       Content="Enabled"
-                      IsChecked="True" />
+                      IsChecked="True"
+                      IsThreeState="True" />
             <CheckBox Margin="{StaticResource ControlMargin}"
                       Content="Enabled"
                       IsChecked="{x:Null}"
-                      IsThreeState="True" />
+                      IsThreeState="True"
+                      Style="{StaticResource MahApps.Styles.CheckBox.Win10}" />
             <CheckBox Margin="{StaticResource ControlMargin}"
                       Content="Disabled"
                       IsEnabled="False" />

src/MahApps.Metro/Controls/Helper/CheckBoxHelper.cs

@@ -0,0 +1,315 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Media;
+
+namespace MahApps.Metro.Controls
+{
+    public static class CheckBoxHelper
+    {
+
+        public static readonly DependencyProperty CheckBoxSizeProperty = DependencyProperty.RegisterAttached("CheckBoxSize", typeof(double), typeof(CheckBoxHelper), new FrameworkPropertyMetadata(18.0));
+
+        /// <summary>
+        /// Gets the size of the CheckBox itself.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static double GetCheckBoxSize(DependencyObject obj)
+        {
+            return (double)obj.GetValue(CheckBoxSizeProperty);
+        }
+
+        /// <summary>
+        /// Sets the size of the CheckBox itself.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static void SetCheckBoxSize(DependencyObject obj, double value)
+        {
+            obj.SetValue(CheckBoxSizeProperty, value);
+        }
+
+
+        #region Checked
+        // Using a DependencyProperty as the backing store for CheckedGlyph.  This enables animation, styling, binding, etc...
+        public static readonly DependencyProperty CheckedGlyphProperty = DependencyProperty.RegisterAttached("CheckedGlyph", typeof(object), typeof(CheckBoxHelper), new FrameworkPropertyMetadata());
+        public static readonly DependencyProperty CheckedGlyphTemplateProperty = DependencyProperty.RegisterAttached("CheckedGlyphTemplate", typeof(DataTemplate), typeof(CheckBoxHelper), new FrameworkPropertyMetadata());
+        public static readonly DependencyProperty CheckedBackgroundBrushProperty = DependencyProperty.RegisterAttached("CheckedBackgroundBrush", typeof(Brush), typeof(CheckBoxHelper), new FrameworkPropertyMetadata());
+        public static readonly DependencyProperty CheckedBorderBrushProperty = DependencyProperty.RegisterAttached("CheckedBorderBrush", typeof(Brush), typeof(CheckBoxHelper), new FrameworkPropertyMetadata());
+
+
+        /// <summary>
+        /// Gets the the Glyph for IsChecked = true.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static object GetCheckedGlyph(DependencyObject obj)
+        {
+            return (object)obj.GetValue(CheckedGlyphProperty);
+        }
+
+        /// <summary>
+        /// Sets the the Glyph for IsChecked = true.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static void SetCheckedGlyph(DependencyObject obj, object value)
+        {
+            obj.SetValue(CheckedGlyphProperty, value);
+        }
+
+
+        /// <summary>
+        /// Gets the the Background for IsChecked = true.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static Brush GetCheckedBackgroundBrush(DependencyObject obj)
+        {
+            return (Brush)obj.GetValue(CheckedBackgroundBrushProperty);
+        }
+
+        /// <summary>
+        /// Sets the the Background for IsChecked = true.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static void SetCheckedBackgroundBrush(DependencyObject obj, Brush value)
+        {
+            obj.SetValue(CheckedBackgroundBrushProperty, value);
+        }
+
+        /// <summary>
+        /// Gets the the GlyphTemplate for IsChecked = true.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static DataTemplate GetCheckedGlyphTemplate(DependencyObject obj)
+        {
+            return (DataTemplate)obj.GetValue(CheckedGlyphTemplateProperty);
+        }
+
+        /// <summary>
+        /// Sets the the GlyphTemplate for IsChecked = true.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static void SetCheckedGlyphTemplate(DependencyObject obj, DataTemplate value)
+        {
+            obj.SetValue(CheckedGlyphTemplateProperty, value);
+        }
+
+        /// <summary>
+        /// Gets the the BorderBrush for IsChecked = true.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static Brush GetCheckedBorderBrush(DependencyObject obj)
+        {
+            return (Brush)obj.GetValue(CheckedBorderBrushProperty);
+        }
+
+        /// <summary>
+        /// Sets the the BorderBrush for IsChecked = true.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static void SetCheckedBorderBrush(DependencyObject obj, Brush value)
+        {
+            obj.SetValue(CheckedBorderBrushProperty, value);
+        }
+
+        #endregion
+
+
+        #region UnChecked
+        // Using a DependencyProperty as the backing store for UnCheckedGlyph.  This enables animation, styling, binding, etc...
+        public static readonly DependencyProperty UnCheckedGlyphProperty = DependencyProperty.RegisterAttached("UnCheckedGlyph", typeof(object), typeof(CheckBoxHelper), new FrameworkPropertyMetadata());
+        public static readonly DependencyProperty UnCheckedGlyphTemplateProperty = DependencyProperty.RegisterAttached("UnCheckedGlyphTemplate", typeof(DataTemplate), typeof(CheckBoxHelper), new FrameworkPropertyMetadata());
+        public static readonly DependencyProperty UnCheckedBackgroundBrushProperty = DependencyProperty.RegisterAttached("UnCheckedBackgroundBrush", typeof(Brush), typeof(CheckBoxHelper), new FrameworkPropertyMetadata());
+        public static readonly DependencyProperty UnCheckedBorderBrushProperty = DependencyProperty.RegisterAttached("UnCheckedBorderBrush", typeof(Brush), typeof(CheckBoxHelper), new FrameworkPropertyMetadata());
+
+
+        /// <summary>
+        /// Gets the the Glyph for IsChecked = false.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static object GetUnCheckedGlyph(DependencyObject obj)
+        {
+            return (object)obj.GetValue(UnCheckedGlyphProperty);
+        }
+
+        /// <summary>
+        /// Sets the the Glyph for IsChecked = false.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static void SetUnCheckedGlyph(DependencyObject obj, object value)
+        {
+            obj.SetValue(UnCheckedGlyphProperty, value);
+        }
+
+
+        /// <summary>
+        /// Gets the the BackgroundBrush for IsChecked = false.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static Brush GetUnCheckedBackgroundBrush(DependencyObject obj)
+        {
+            return (Brush)obj.GetValue(UnCheckedBackgroundBrushProperty);
+        }
+
+        /// <summary>
+        /// Sets the the BackgroundBrush for IsChecked = false.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static void SetUnCheckedBackgroundBrush(DependencyObject obj, Brush value)
+        {
+            obj.SetValue(UnCheckedBackgroundBrushProperty, value);
+        }
+
+        /// <summary>
+        /// Gets the the GlyphTemplate for IsChecked = false.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static DataTemplate GetUnCheckedGlyphTemplate(DependencyObject obj)
+        {
+            return (DataTemplate)obj.GetValue(UnCheckedGlyphTemplateProperty);
+        }
+
+        /// <summary>
+        /// Sets the the GlyphTemplate for IsChecked = false.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static void SetUnCheckedGlyphTemplate(DependencyObject obj, DataTemplate value)
+        {
+            obj.SetValue(UnCheckedGlyphTemplateProperty, value);
+        }
+
+        /// <summary>
+        /// Gets the the BorderBrush for IsChecked = false.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static Brush GetUnCheckedBorderBrush(DependencyObject obj)
+        {
+            return (Brush)obj.GetValue(UnCheckedBorderBrushProperty);
+        }
+
+        /// <summary>
+        /// Sets the the BorderBrush for IsChecked = false.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static void SetUnCheckedBorderBrush(DependencyObject obj, Brush value)
+        {
+            obj.SetValue(UnCheckedBorderBrushProperty, value);
+        }
+
+        #endregion
+
+
+        #region Intermediate
+        // Using a DependencyProperty as the backing store for IntermediateGlyph.  This enables animation, styling, binding, etc...
+        public static readonly DependencyProperty IntermediateGlyphProperty = DependencyProperty.RegisterAttached("IntermediateGlyph", typeof(object), typeof(CheckBoxHelper), new FrameworkPropertyMetadata());
+        public static readonly DependencyProperty IntermediateGlyphTemplateProperty = DependencyProperty.RegisterAttached("IntermediateGlyphTemplate", typeof(DataTemplate), typeof(CheckBoxHelper), new FrameworkPropertyMetadata());
+        public static readonly DependencyProperty IntermediateBackgroundBrushProperty = DependencyProperty.RegisterAttached("IntermediateBackgroundBrush", typeof(Brush), typeof(CheckBoxHelper), new FrameworkPropertyMetadata());
+        public static readonly DependencyProperty IntermediateBorderBrushProperty = DependencyProperty.RegisterAttached("IntermediateBorderBrush", typeof(Brush), typeof(CheckBoxHelper), new FrameworkPropertyMetadata());
+
+
+        /// <summary>
+        /// Gets the the Glyph for IsChecked = null.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static object GetIntermediateGlyph(DependencyObject obj)
+        {
+            return (object)obj.GetValue(IntermediateGlyphProperty);
+        }
+
+        /// <summary>
+        /// Sets the the Glyph for IsChecked = null.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static void SetIntermediateGlyph(DependencyObject obj, object value)
+        {
+            obj.SetValue(IntermediateGlyphProperty, value);
+        }
+
+
+        /// <summary>
+        /// Gets the the BackgroundBrush for IsChecked = null.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static Brush GetIntermediateBackgroundBrush(DependencyObject obj)
+        {
+            return (Brush)obj.GetValue(IntermediateBackgroundBrushProperty);
+        }
+
+        /// <summary>
+        /// Sets the the BackgroundBrush for IsChecked = null.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static void SetIntermediateBackgroundBrush(DependencyObject obj, Brush value)
+        {
+            obj.SetValue(IntermediateBackgroundBrushProperty, value);
+        }
+
+        /// <summary>
+        /// Gets the the GlyphTemplate for IsChecked = null.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static DataTemplate GetIntermediateGlyphTemplate(DependencyObject obj)
+        {
+            return (DataTemplate)obj.GetValue(IntermediateGlyphTemplateProperty);
+        }
+
+        /// <summary>
+        /// Sets the the GlyphTemplate for IsChecked = null.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static void SetIntermediateGlyphTemplate(DependencyObject obj, DataTemplate value)
+        {
+            obj.SetValue(IntermediateGlyphTemplateProperty, value);
+        }
+
+        /// <summary>
+        /// Gets the the BorderBrush for IsChecked = null.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static Brush GetIntermediateBorderBrush(DependencyObject obj)
+        {
+            return (Brush)obj.GetValue(IntermediateBorderBrushProperty);
+        }
+
+        /// <summary>
+        /// Sets the the BorderBrush for IsChecked = null.
+        /// </summary>
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(CheckBox))]
+        public static void SetIntermediateBorderBrush(DependencyObject obj, Brush value)
+        {
+            obj.SetValue(IntermediateBorderBrushProperty, value);
+        }
+
+        #endregion
+
+    }
+}