[Development] Move math3d from QtGui to QtCore

[Laszlo Papp]

> Either you haven't looked at Eigen, or you haven't understood it.

You can say that these are wrongly packaged things, but it is an
additional dependency for getting things built on gentoo (and source
distributions in general) anyway:
http://www.archlinux.org/packages/extra/any/eigen2/

Also, why would I look into a more difficult API with a lot of
features around I do not need, especially if we already have a good
deal of work done in Qt? Again, I prefer the Qt ecosystem. Perhaps, I
am wrong about that, especially if I even worked on that? I will have
a local copy in the end for maintenance anyway internally.

BR,
Laszlo Papp