Make it possible to put QObject tracked with QWeakPointer inside QSharedPointer

Make it possible to put QObject tracked with QWeakPointer inside QSharedPointer

Do that by keeping the QWeakPointer that track QObject independent of
the ones that track QSharedPointer.

QSharedPointer do not touch the sharedRefCount in QObjectPrivate anymore

When converting a QWeakPointer constructed from a QObject to a
QSharedPointer, it will display a warning saying one should not do that.

Task-number: QTBUG-22622
Change-Id: I3595e3e7401702410776c458687ab357ad9366ab
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>