Add new attached property TreeViewItemHelper.ToggleButtonStyle

[punker76]

Add this new attached property to make it possible to set the inner ToggleButton style. The default is MahApps.Styles.ToggleButton.ExpandCollapse.

Sample

<Style BasedOn="{StaticResource MahApps.Styles.TreeViewItem}" TargetType="{x:Type TreeViewItem}">
    <Setter Property="controls:TreeViewItemHelper.ToggleButtonStyle">
        <Setter.Value>
            <Style BasedOn="{StaticResource MahApps.Styles.ToggleButton.ExpandCollapse}" TargetType="{x:Type ToggleButton}">
                <Setter Property="Visibility" Value="Hidden" />
            </Style>
        </Setter.Value>
    </Setter>
</Style>

or

<Style x:Key="SpecialCustomTreeViewItemToggleButtonStyle" BasedOn="{StaticResource MahApps.Styles.ToggleButton.ExpandCollapse}" TargetType="{x:Type ToggleButton}">
    <Setter Property="Visibility" Value="Hidden" />
</Style>

<Style BasedOn="{StaticResource MahApps.Styles.TreeViewItem}" TargetType="{x:Type TreeViewItem}">
    <Setter Property="controls:TreeViewItemHelper.ToggleButtonStyle" Value="{StaticResource SpecialCustomTreeViewItemToggleButtonStyle}"  />
</Style>

or

<TreeView Margin="2">
    <TreeViewItem Header="Item 1"
                  Controls:TreeViewItemHelper.ToggleButtonStyle="{StaticResource SpecialCustomTreeViewItemToggleButtonStyle}">
        <TreeViewItem Header="Item 1.1">
            <TreeViewItem Header="Item 1.1.1" />
            <TreeViewItem Header="Item 1.1.2" />
        </TreeViewItem>
        <TreeViewItem Header="Item 1.2" />
        <TreeViewItem Header="Item 1.3" />
    </TreeViewItem>
</TreeView>