gui: fix key binding in alignment tabs

some axes have been tweaked to be inverted, and the keyboard bindings need to be inverted too.
gui: small clean ups
gui model: change tab switching to use a VA

Instead of having a special singleton (tab controller) and a fake list, use a VA,
more or less similarly to .focussedView.

We even go further with .tab to also contain all the available tabs on .tab.
Probably we could do the same on .focussedView/.views.

The main problem of this new solution is that the model is a bit "dirtied" by referencing a Tab,
which is a not a model object.

Directly use the new VA to fix a bug in snapshot: acquisition is now of the _current_ tab.