core: Fix execution order

[relrelb]

This is a prototype PR for executing actions in BFS order, rather than DFS order.
Fixes at least #1626 (now Sonic runs through the corkscrew loop) and #1986 (now enemies get hit).
Doesn't fix other frame-order-execution bugs such as #1164 and #1915. They might or might not be related.

[Herschel]

I believe the execution order list is actually a single global list containing all clips. The order of action execution is based on the creation order of the clips, not hierarchal -- clips get tagged onto front of the list as they are created.

Here's a sample:
execorder.zip
The final order is "clip3, clip2, clip1, root", with the newest clip first (this remains the same no matter the hierarchy I believe).

Some of the details might be wrong, but this is my understanding from various tests. Having a single list would also make some things easier (for example, it is easier to implement broadcast events in both AVM1/AVM2 because we can simply iterate over the list).

[relrelb]

Closing in favor of a better upcoming approach.