QWeakPointer can do the same, so there's no reason to not allow it for
QPointer.
[ChangeLog][QtCore][QPointer] QPointer<T> can now be (move- and
copy-)constructed from QPointer<X>.
Fixes: QTBUG-112464
Change-Id: I77cf5d39974bf2b3ec849b4afc33e286e864821e
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>