appDisplay: Don't expose "Add as favorite" for window-backed apps

We don't know how to do it.  Similarly, don't allow New Window.

https://bugzilla.gnome.org/show_bug.cgi?id=648149

js/ui/appDisplay.js

@@ -673,6 +673,7 @@ AppIconMenu.prototype = {
             item._window = windows[i];
         }
 
+        if (!this._source.app.is_window_backed()) {
             if (windows.length > 0)
                 this._appendSeparator();
 
@@ -683,7 +684,7 @@ AppIconMenu.prototype = {
 
             this._toggleFavoriteMenuItem = this._appendMenuItem(isFavorite ? _("Remove from Favorites")
                                                                 : _("Add to Favorites"));
-
+        }
     },
 
     _appendSeparator: function () {