stage: Normalize key focus setting

We often mean that when key_focus == NULL, it's assumed to be on the
stage, and clutter_stage_get_key_focus() reflects this. We also do a
lot of check around the lines of key_focus == NULL instead of also
checking for the stage, so make sure to normalize it so that explicitly
grabbing the stage's key focus will not change our behaviour.

https://bugzilla.gnome.org/show_bug.cgi?id=683301

clutter/clutter-stage.c

@@ -3039,6 +3039,10 @@ clutter_stage_set_key_focus (ClutterStage *stage,
 
   priv = stage->priv;
 
+  /* normalize the key focus. NULL == stage */
+  if (actor == CLUTTER_ACTOR (stage))
+    actor = NULL;
+
   /* avoid emitting signals and notifications if we're setting the same
    * actor as the key focus
    */