MenuItem: use queued connection to trigger() - avoid deletion crash

If a ListView's delegate launches a context menu which deletes the
corresponding model item, the MouseArea which launched the context menu
gets deleted while handling an event.  That is not allowed, so a
mechanism is necessary to postpone the deletion.  This works for now.

Task-number: QTBUG-36653
Change-Id: Idb78ddb830f229e2006f53fc993f1aee8fb2dc15
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>