Blender Git Loki

Fix T41467: Modifier view buttons changing positions.

Commits early in this year (to save some space) broke this. Hopefully this time
it works in all cases - lastCageIndex is no more influenced by realtime/edit active states.
Also, inactivate buttons instead of hiding them, can be useful to set those data
even though it does not have any immediate effect.

Took the opportunity to switch cage buttons to RNA, btw.