Update Xwt dependencies for XS 4.1.9

Building with the latest alpha of Xamarin Studio (4.1.9) gives the following error:

The module/namespace 'Xwt.Motion' from compilation unit 'Xwt' did not contain the namespace, module or type 'IAnimatable'

This pull request updates Xwt.dll and Xwt.Gtk.dll to the same versions used in 4.1.9, so we can build again. This may break compat with 'Stable' channel, so maybe it is too early for this?