window-actor: Set every window actor to be reactive

Now that the background actor is reactive, this means that
clicks on the window group part of the stage, even when they're
on an X window, will be registered as the background actor, as
all of the other children of the group aren't reactive. This can
happen when a plugin takes a modal grab, for instance.

https://bugzilla.gnome.org/show_bug.cgi?id=681540

src/compositor/meta-window-actor.c

@@ -1576,6 +1576,8 @@ meta_window_actor_new (MetaWindow *window)
 
   clutter_actor_hide (CLUTTER_ACTOR (self));
 
+  clutter_actor_set_reactive (CLUTTER_ACTOR (self), TRUE);
+
   /* Initial position in the stack is arbitrary; stacking will be synced
    * before we first paint.
    */