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/workspace.js

@@ -602,7 +602,7 @@ Workspace.prototype = {
         this.actor.height = global.screen_height;
         this.scale = 1.0;
 
-        let windows = global.get_windows().filter(this._isMyWindow, this);
+        let windows = global.get_window_actors().filter(this._isMyWindow, this);
 
         // Find the desktop window
         for (let i = 0; i < windows.length; i++) {