Qt Quick 3D: C++ API

So far all of the public API in Qt Quick 3D is QML API (hence the name).  However we should expose parts of the C++ API to make it possible to create custom content similarly to how the public Qt Quick Scenegraph API works in Qt Quick today.

It's still yet to be seen how much actually needs to be exposed because we need to validate it with some C++ examples.

Also the plan is to have QQuick3DRenderNode inherit from a common base-class as QSGNode so we will need to get a bit further on before we can know all of the details of how this will work.  For now the backend nodes are designed by compatible with QSGNode by using the same patterns as we do in Qt Quick.

As far as a more generic C++ API to say: do everything in C++.  I would say that is possible, but we are much further from that today given the current quality of the API which we consider private.  This needs to be looked into though.