[WPF] TreeView fixed wrong Shift selecting + implemented Keyboard support
Improve cell view backend api

Now there is a ICellViewFrontend interface for
each cell view which provides access to features
which are only required by cell view implementations
(such as raising events)
Added GetBackend helper method
[WPF] Track api changes
[Xwt.Mac] Fix ToCGColor utility method to use the same colorspace than ToNSColor.

CGColor and our NSColor defaults to two different colorspaces which produces different colors when displayed on screen.
[Xwt.Mac] Make Xwt.Mac widgets uses CALayer and provide a default implementation of BackgroundColor using it.
[Xwt] Backport performance improvements to Table
[Xwt.Mac] Don't scale images in NSButtons and set the mode to image-only when there is no label
[Xwt.Mac] Improve positioning of spinner subviews since Xwt layout seems to not play nice with the default autoresize mask.
Improve image size handling

The Size property now returns the value of GetDefaultSize
if no size is explicitly set.
[Xwt.Mac] Only enable CALayer when the general implementation of BackgroundColor is called.
[Gtk] Fix a typo in CellUtil.CreateCellRenderer that caused an InvalidCastException in the test app
Added docs to the Image class
Add api for rendering a widget into a bitmap
Add scrollbar widget
[GTK] Implement scrollbar widget backend
Add scrollbar widget sample
[Mac] Add initial scrollbar implementation
[Mac] Fix ListBox backend
Scrollbar sample fixes