Add Code Completion for AppController fields

[junichi11]

Currently, Code Completion for helpers, components, e.t.c. fields is available only concrete controller.
e.g.
If we set helpers to AppController, code completion for helper is not available.

class AppController extends Controller {
    public $helpers = array('Foo', 'Bar'); // these helpers are not available in view
}

class PostsController extends Controller {
    public $helpers = array('MyHtml', 'MySession'); // these helpers are available in view
}