Fix for bug #220. Xwt.Gtk editable checkbox cell views did not work properly.
Merge https://github.com/mono/xwt into HEAD
Merge pull request #222 from mono-soc-2013/luiscubal-fix-checkbox

Fix editable Xwt.Gtk checkbox cell views
[GTK] Properly fix the checkbox cell view
Add some docs
Added TextChanged event to TextCellView
[GTK] Properly get and set tree model values

Factorized the methods for getting and setting tree model values, so that
they can be used in all cell implementations.
This fixes a dbnull cast exception in when updating values of a cell.
Add animation classes
Add animation classes
Add support for widget and window opacity
[WPF] Add opacity support
Fix NRE when getting the primary screen
[GTK] Fix primary screen getter
Added property for setting the initial location of a window
[Mac] fix link label breakage after e7586f3e35c1250116015a14547eccd18e3d1fd4