[Development] Move math3d from QtGui to QtCore

[Laszlo Papp]

> No, we're not. I asked if the classes were enough and the answer I received
> was that they aren't.

I believe, I am not following you. It is enough for my case as of now,
but if that is blocked into QtCore for whatever reasons, and if there
is a need for more in the same module like Uwe requested, we would
still need a new module either way, anyway.

In addition, I find QPointF3D important as well. Perhaps at least that
class along with QPointF in QtCore, or not even that?

Best Regards,
Laszlo Papp