[Development] Move math3d from QtGui to QtCore

[Laszlo Papp]

> OpenGL, Audio 3D, QtSensors, Qwt, and others are not really just "my
> project" who spoke up to have something common.

To be fair, "my project (just a Qt add-on)" is a bit misleading
"qualifier". Ideally, a library (also this one) would be used by many
applications. So any hard coded decision would take effect on *every*
application. Hence, even if it was only one library, the end result
would mean many applications, or even higher-level libraries built on
top of (this is why QtGui is never gonna be a dependency for
QtAudio3D).

Best Regards,
Laszlo Papp