If the menu is closed from the keyboard gesture, and
the focus object doesn't change, the menu will still
be in a visible state, even if the keyboard is hidden.

This patch will ensure that this can not be the case
by listening for keyboardWillHideNotification. Since
we have no guarantee for when the destructor runs, we
apply a pessimistic approach and ensure we stop listen
when the menu gets closed.

Task-number: QTBUG-42523
Change-Id: If734ea32d1823b978c9c1c67ebcc5b6c3c5c338c
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>