Blender Git Loki

Fix T62321: Crash on motion tracking when dopesheet open and deleting track

Need to ensure all dopesheet are properly tagged for update.

The tricky part here is that due to the animation we need to keep
movie clips covered by copy-on-write, but the interface uses an
original clip to draw dopesheet.

Use same idea of an active dependency graph as is done in other
similar cases.