Arbitrary slicing, 2nd attempt.

- Instead of rotation around z-axis followed by rotation around y-axis we now rotate around a single newly calculated axis on every node placement.
- TODO: Currently rotation angle is calculated based on the last 5 recentering positions. Usability test needed here.
- TODO: Buttons in skeleton vp to face xy-/xz-/yz-plane should be adapted to arbitrary slicing to face the rotated vps respectively.
- While fixing rotation, I discovered a bug concerning texture loading in arbitrary mode: hold key K (rotation around z-axis) and you'll notice that in the lower left vp the centered vertical stripe remains black at some angles.