[GTK] fix RemoveSignalHandler extension method
Merge branch 'extend-webview'

Conflicts:
	Xwt.Gtk/Xwt.Gtk.csproj
	Xwt.Gtk/Xwt.GtkBackend/GtkInterop.cs
	Xwt.Gtk/Xwt.GtkBackend/GtkWorkarounds.cs