Support multiple named animations per Tile

I would really like to have multiple (named) animations on one tile, this is especially useful for objects, since these might change their animation during the game.
Some examples:
- Collectibles that make a little splosh when collected
- Enemies that can walk, stand, die, ...
- A Goal that shows a nice animation when reached
It would be great to have all these animation specifications in a single place, and I think the Tiled file would be an excellent location.

For the time being I will probably create each animation on the starting tile and add the names as custom properties.

Start with issue #986 and review the existing custom-property workaround described in its body. Trace how tile animation specifications are represented and edited, then define completion around storing multiple named animations in one Tiled file for objects that change animation during gameplay.

Written by the indexing model from the issue text.