Adapt to removal of Mutter namespace

MutterWindow and MutterPlugin have been renamed to MetaWindowActor
and MetaPlugin, mutter_plugin_list_windows() to
meta_plugin_list_window_actors(). Adapt to those changes.

https://bugzilla.gnome.org/show_bug.cgi?id=632500

js/ui/windowManager.js

@@ -415,7 +415,7 @@ WindowManager.prototype = {
             return;
         }
 
-        let windows = global.get_windows();
+        let windows = global.get_window_actors();
 
         /* @direction is the direction that the "camera" moves, so the
          * screen contents have to move one screen's worth in the