QCocoaMenu: Have the menu delegate return the right amount of items

In some cases we move menu items around, therefore we should
not rely on QCocoaMenu::items() being the actual items inside
the menu. But, since the NSMenu is updated before hand, we can
rely on NSMenu.numberOfItems instead.

Change-Id: Icd4497beca4f52a6d38408eeaa2e6ec71b579685
Task-number: QTBUG-52931
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>