Allow passing qmlRegisterSingletonType QObjects to C++ as QObject*

At the moment you can pass them as their FinalType* or
as one of their ParentType* but not as QObject* which
does not make much sense to me

Task-number: QTBUG-34617
Task-number: QTBUG-30730

Change-Id: Id5cfb7bbb123456ef43f44f33b450f8966a7641a
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>