Make checking if an item is under the pointer asynchonous again

Checking if an item is under the pointer by calling stage.get_actor_at_pos()
synchronously will trigger a too-early allocation of the stage. Use an idle
at Meta.PRIORITY_BEFORE_REDRAW. (Before 553503da it was using a 5 msec timeout,
553503da made it synchronous.)

http://bugzilla.gnome.org/show_bug.cgi?id=592608