[Development] Move math3d from QtGui to QtCore

[Laszlo Papp]

> moving the math3d classes to a more prominent place instantly prompts
> the question: why don't we just use Eigen?

The question instantly prompts the answer: eigenX is not part of the
Qt ecosystem (with all its traits, what that means), and we are just
discussing few already existing classes in the qt-project, and not
third-party full-fledged projects.

BR,
Laszlo Papp

PS.: We used to use eigen two years ago in a project based on top of
Qt, before switching this math3d solution...