Floating watermarks now trigger better

[Dotnetifier]

Fixes #2225

List of changes:

• The PART_FloatingMessageContainer grid is now a ContentControl because the layout system for ContentControl is way simpler and therefore faster. Also, the grid wasn't necessary.
• The triggers that active the hide/show of the floating watermark are no longer three separate DataTriggers. This system now consists of a MultiDataTrigger with two conditions, which both must be satisfied:
  • Whether the input box should have a floating watermark
  • Whether the input box has text
  • The third one, whether the input box actually has a value for TextBoxHelper.Watermark has been dropped, because there are no 'negative' DataTriggers. This brings me to the next change:
• The animated property is no longer Height; it is now MaxHeight. If the watermark is not set, the TextBlock will not take any space because it uses an added MetroAutoCollapsingTextBlock style, which is based on MetroTextBlock and will automatically collapse when the text property is empty.

[punker76]

@Dotnetifier thx!

[Dotnetifier]

That's fine. I already found a way to solve #2235 when I created this PR, but I'm too much of a git noob to do too many branches and stuff at once 😒

[punker76]

@Dotnetifier i solved it while testing your changes :-D

[Dotnetifier]

Doesn't that make PART_WatermarkMessage at 471 redundant, if the (floating) watermark responsibility is passed on to PART_EditableTextBox at 451?

[punker76]

@Dotnetifier PART_WatermarkMessage is for non editing combobox

[Dotnetifier]

@punker76 Ah I see, thanks