messageTray: Clone, clip & move the window_group when tray is toggled

1) straddling windows get clipped at the monitor boundary
2) we move the bottom monitor and not the primary because that is
   where the tray is
3) to stop the wallpaper from the bottom monitor leaking into the
   primary, we adjust the clip as the clone animates up/down

https://bugzilla.gnome.org/show_bug.cgi?id=681392