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/workspacesView.js

@@ -190,7 +190,7 @@ GenericWorkspacesView.prototype = {
     },
 
     _onRestacked: function() {
-        let stack = global.get_windows();
+        let stack = global.get_window_actors();
         let stackIndices = {};
 
         for (let i = 0; i < stack.length; i++) {