WIP: Auto transitions
Two tests which illustrate that when loading this specific project:
- layer2 has auto-transition False instead of True.
- the two clips on layer1 overlap but don't form a transition as they should.
This weirdness will probably go away when we fail project loading in case three or more clips overlap at the same position. But still maybe we want to see what's going on, maybe we get some insight about how to do this restriction better.