[ShellGtkEmbed] do a better job of tracking the actor's position

Previously the code to keep the X window aligned with its actor made
various assumptions that were true in the panel status tray area but
that are not true in the message tray. Fix it up by repositioning the
X window at clutter_actor_paint() time, which will definitely be
called every time the actor moves, whether it's because of its own or
its parent/ancestor's changes.

https://bugzilla.gnome.org/show_bug.cgi?id=608869