app-menu: Fix quit menu item

As popup menus now take focus for keyboard navigation, no application
is focused when activating the menu. Use the target application instead,
which keeps track of the application currently associated with the menu.

https://bugzilla.gnome.org/show_bug.cgi?id=634103

js/ui/panel.js

@@ -408,9 +408,9 @@ AppMenuButton.prototype = {
     },
 
     _onQuit: function() {
-        if (this._focusedApp == null)
+        if (this._targetApp == null)
             return;
-        this._focusedApp.request_quit();
+        this._targetApp.request_quit();
     },
 
     _onAppStateChanged: function(tracker, app) {