[Development] Move math3d from QtGui to QtCore

[Laszlo Papp]

> 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.

I am sorry for the delay with this, but I have not had stable internet
connection recently, and was traveling a lot. So, here is the change
for 5.1 then, if this cannot get into 5.0:
https://codereview.qt-project.org/#change,30407

Best Regards,
Laszlo Papp