stage: set key focus actor to NULL before emitting focus-out

stage: set key focus actor to NULL before emitting focus-out

Someone might hide the previously focused actor in the focus-out signal
handler and as key focus still appears to be on that actor we'd get
re-entrant call and get glib critical from g_object_weak_unref

This changes clutter_stage_get_key_focus() to return stage/NULL during
focus-out signal emission. It used to be the actor focus-out was being
emitted on.

Fixes bug:

  http://bugzilla.openedhand.com/show_bug.cgi?id=1547

Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>