Merge branch 'master' of https://github.com/mono/xwt
[GTK] add initial Gtk3 support
[GTK] add Gtk3 support for ISearchTextEntryBackend
[GTK] honor size requests with Gtk3
[GTK] fix CanvasBackend sizing issues with Gtk3
[GTK] fix Gtk3 dll names for Windows
Merge branch 'gtk-fixes' into gtk3
[GTK] rename changed Gtk3 TreeModel interfaces to reduce XWT_GTK3 usage
[GTK] add toolkit (gtk2/3) specific extensions to reduce XWT_GTK3 usage
[GTK] move Gtk3 ISearchTextEntryBackend impl to a separate file
[GTK] add a custom Gtk.CellRenderer base with toolkit (gtk2/3) specific implementations
[GTK] revert dangerous try/catch block
[GTK] add a custom GtkViewPort base for CustomViewPort with toolkit (gtk2/3) specific implementations
CustomViewPort must be sealed because of the new adjustment properties for Gtk3
[GTK] add a custom GtkDrawingArea base for ImageBox with toolkit (gtk2/3) specific implementations
Merge remote-tracking branch 'origin/add-text-cursor-selection' into gtk3
[GTK] move toolkit (gtk2/3) specific backend code to separate files
[GTK3] add Widget Opacity support
Merge remote-tracking branch 'upstream/master' into gtk3

Conflicts:
	TestApps/Samples/Samples/TextEntries.cs
	Xwt.Gtk/Xwt.GtkBackend/CanvasBackend.cs
[GTK] optimize BackgroundColor handling
[GTK3] use cairo context provided by Gtk instead of creating a new one for canvas drawing