[FEATURE] Gimbal rotation

[GilesBathgate]

Is your feature request related to a problem? Please describe.
Arcball rotation solves the issue of gimbal lock, but it's not really very "nice" for moving the printer around. Essentially the camera roll isn't locked, so you easily end up with a printer that isn't level to the horizon.

Describe the solution you'd like
Gimbal based rotation is quite simple, here is my implementation: https://github.com/GilesBathgate/RapCAD/blob/master/src/ui/glview.cpp#L463 (shameless plug)
Yes, you can get into a gimbal lock situation, but it's not common in practice.

Describe alternatives you've considered
Make it optional/user preference?

[vintagepc]

I would support this, there's no real reason we went with Arcball other than that it was convenient and had a nice ready-to-go third party implementation we could just drop in.