[Development] Move math3d from QtGui to QtCore

[Laszlo Papp]

> I don't see a problem with a QPoint3D and QPointF3D class in QtCore, in
> Qt 5.1, provided you also fix any ambiguities with QVector3D's existence.

Okay, I will try to put up something soon with WIP then, based
somewhat on the current QPoint(F) implementations. I will also double
check the Qwt needs in order to make sure we fulfill those as well.

Bst Regards,
Laszlo Papp