Fix loading custom displayComponent inside ActionToolBar sub actions

The issue is that due to the Loader, it was not possible to close automatically the menu once clicking on an menu item.

This is an API break because the previous api accepted everything but didn't really work. Now only elements who have a action property are accepted but in really the menu will only work correctly with QQC2.MenuItem.

This can be tested with: plasma/plasma-workspace!1391 (merged)