Modelviewer fixes

Mostly input stuff.

* Increase the far plane to make large models visible (#1933).
* Use right shift for small increase in zoom/move speed, left for large.
* Add alternate key cluster for keyboards without a numpad.
* Switch to turntable-style mouse rotation.
* Zoom with mouse wheel.

More things to do for next time:

* Allow trackball/arcball-style rotation as an option.
* More UI for some of the hidden features (wireframe, random colours, etc)