Remove usage of MetaScreen

Remove any usage of MetaScreen, as it has been removed from libmutter
in the API version 3. The corresponding functionality has been moved
into three different places: MetaDisplay, MetaX11Display (for X11
specific functionality) and MetaWorkspaceManager.

https://bugzilla.gnome.org/show_bug.cgi?id=759538

js/ui/appDisplay.js

@@ -1862,7 +1862,8 @@ var AppIconMenu = new Lang.Class({
 
         // Display the app windows menu items and the separator between windows
         // of the current desktop and other windows.
-        let activeWorkspace = global.screen.get_active_workspace();
+        let workspaceManager = global.workspace_manager;
+        let activeWorkspace = workspaceManager.get_active_workspace();
         let separatorShown = windows.length > 0 && windows[0].get_workspace() != activeWorkspace;
 
         for (let i = 0; i < windows.length; i++) {