Refactor Creature animation logic.

Move most of the logic for creating a Creature’s Animation into the
Creature class, and give the animation data its own class. This should
allow much more easily customizable Creature “behavior.”
Do more of the work for percussion sounds in MidiPercussion.
Add helper functions for spinning of a thread to play a NoteContainer.
Remove debugging prints.
Remove now-unused imports.
Move animation logic from Creature into Beastie.

I’m pretty sure all this logic will be Beastie-specific.
Store beat_length as a float.

Other things use it for timing so it’s easier not to have to explicitly
convert it to a float all the time.
Preliminary logic for turns; allow flash to happen on a beat other than the first.