messageTray: Don't use focus grabs

We can easily implement much of the same behavior ourselves by
keeping track of Clutter's focus events. Reintroduce heavily
modified FocusGrabber to do the work for us.

This will temporarily break when the user selects a window until
we can make gnome-shell automatically set the stage focus.

This also removes our only use of focus grabs, so remove those
as well.

https://bugzilla.gnome.org/show_bug.cgi?id=700735