Change animation API.

This should have been broken into more commits, but here we are:
Rename <animation-state> to <animator>. Better name, less verbose.
Make animators mutable because it makes more sense than when
animation-states where immutable.
Update sprite module to reflect the animation API changes.
Add the playing flag back to animator objects.
Clean up parameters referring to old type name.
Add agenda-schedule-interval procedure.
Animate sprites during game updates instead of on render.

Frame rate is independent of the update rate, so the old way didn't
make sense.