Add settings for specific sprite animations

[kxcrl]

Allow people to add arbitrary events and then jump to an arbitrary Y position and animate across the X axis when the events fire.

Options:

1. Play once before returning
   • A sprite facing to the right at y=1 may have a walking forward animation at y=5. In this case, when moving to the right, it should play the animation at y=5 and then return to y=1 while idle.
2. Play continuously
   • A sprite may have an alternative animation for its current idle state at some other y value that it switches to as the result of an event.
3. Play once and stop
   • A sprite may have a dying animation that it plays once through, halting at the end and no longer animating

[kxcrl]

This may not actually be a good idea, especially early on, since lots of non-standard sprites could end up making things hard to update for people later. Closing for now.