Improve out-of-sequence undo for move events

[Oletus]

Out-of-sequence undo for move events doesn't produce very good results yet.

One reasonable goal for out-of-sequence undo would be that the following sequence would not change the picture:

1. Move event A
2. Move event B
3. Undo move event A
4. Undo move event B

but it's a bit unclear what's the best way to do this. Merges happening between move & undo can also mess things up, that should be thought about.