Remove unnecessary dummy headers for qml classes

QDoc can now deal with QML only classes. We should be able to just put cube.qdoc into Cube.qml (with a simple "\qmlclass Cube").

See http://doc-snapshot.qt-project.org/qdoc/componentset.html for an example of documenting pure QML objects.
Unless the comment in cube.h is misleading, you can then merely exorcise cube.h.

We should also check whether there are more dummy headers for other qml classes and eliminate them, too.