[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