Notifications can't be withdrawn via Gtk.Application

According to http://valadoc.org/#!api=gio-2.0/GLib.Application, notifications should be able to get withdrawn via "widthdraw_notification ()"

It looks like it is removing it from some internal list since more notifications with the same ID will not replace the existing one, but will instead create a new one. But the actual notification does not get removed by the function atm