Enable QWidget based classes to be used in QML files

Currently, QWidget based class cannot be a container for other QWidget object in QML because of Q_ASSERT(!d->isWidget); in object.cpp:1921

To remove this assert or find some better solution, because QWidget base QML component library can be created then and declarative stuff can be exposed to QWidget world.

Maybe there are some circumstances I'm not aware of, but I was playing bit around and this concept seems to work. I have simple application with QWidget derived classes, qml and screen-shot is attached.