Rotation axis should be more intuitive

[hannobraun]

As noted by @chrisprice in #644, the camera rotation can be unintuitive, as the model rotates around a fixed axis, not an axis relative to the camera. This can be reproduced somewhat easily using the test model, by starting a rotation and moving the mouse in a few different directions. It is quite easy to get into a situation, where the direction of the mouse movement no longer has any relation to the direction of the rotation.

It would probably be best to always rotate around an axis relative to the camera, so the rotation always follows the movement of the mouse.