QML should handle QSharedPointer<QObjectDerived> types

It should be possible to read Q_PROPERTY(QSharedPointer<QObjectDerived>) from QML, use setContextProperty with a QVariant containing a QSharedPointer<QObjectDerived> or otherwise make such a type useful in the QML environment.

Q_DECLARE_SMART_POINTER_METATYPE needs to be extended to register a conversion from QSharedPointer<QObjectDerived> to QSharedPointer<QObject> (and perhaps a conversion for std::shared_ptr too).