[WPF] [GTK] Sensitivity for WindowFrame

In Xwt, only Xwt.Widget contains the property Sensitive, and this excludes Xwt.Window, while in both GTK# and WPF, this property is available for Gtk.Widget and System.Windows.UIElement, both in the inheritance path of Window.

On Mac, Window.Sensitive is already in the repository.

This implements #349