Orbital camera should always maintain an up axis internally

[teh-cmc]

The orbital camera works great when you've explicitly logged ViewCoordinates (which has its own issues... #3538), but when you don't, the camera will be in permanent tilt-mode where it is almost impossible to position it the way you'd like as it'll keep tilting when all you're trying to do is rotate around your focus point.

We should have a default up axis when no viewcoordinates are set. It doesn't matter what that up axis is, as long as users can modify it (which they already can using e.g. mouse click... except not really, see next point).

There is another issue though: when users explicitly enter tilt-mode today (e.g. using middle mouse click), we completely drop the notion of an up-axis altogether, and so we enter a weird state where there is no up axis at all and the camera once again becomes very hard to use.

We should always have an up-axis internally (which is either the default one, or one coming from a ViewCoordinates), and middle clicking should modify that up-axis as required.