Merge pull request #245 from steffenWi/master

WPF Bugfix and enhancements
[GTK] Text cell views take all available space

Fixes issue #247
Added Keyboard class

Also refactored conversion methods in GTK backend
[Mac] Implement KeyboardHandler
Fix bug in Color.WithIncreasedContrast
Cope with more build changes
More paths.
Revert "More paths."

This reverts commit ac28d1c25b353ff1b0a52bbf23a185e5369e27a8.
Revert "Cope with more build changes"

This reverts commit 98e2e31ea6cc45d691776891542ffa97b0249722.
[Xwt.WPF] Fix compilation
[Wpf] Implement TextBox/PasswordBox.PlaceholderText

We can implement this using an Adorner. This gives us the
ability to render on top of another widget for the purposes
of adding things like Placeholder Text to widgets which lack
this ability built in.
[WPF] Don't show the dialog separator when no buttons are visible
[Wpf] More fixes for the PlaceholderTextAdorner
[Wpf] Automatically remeasure/repaint when the Image changes

The ImageSource property affects both measure and visual, so invalidate
both of those when the property changes value.
[Wpf] Split the declaration to a new line
[GTK] Canvas now doesn't clear the background

Canvas will now respect the bacground of the container on which it is
rendered as long as no mouse events are required.
Fix creation of backend for custom widgets

Now that backends are specified per-class using the BackendType attribute
there is no need to special case widgets that inherit directly from Widget.
The ICustomWidgetBackend is now assigned to Widget using a BackendType
attribute.
Added FrameBox widget

This widget replaces the CustomFrame type in the Frame widget.
Added support for internal children

Internal children of a container are not included in the children list and
the parent hierarchy chain. Internal children are useful for implementing
containers that depend on other containers to lay out widgets.
[GTK] Fix some allocation issues

Fixed the calculation of the mouse position.
Avoid negative sizes in the canvas widget.