Support more than 1 tileset

[karl-zylinski]

Currently the Tile_IDs look like T0Y0X2 where T0 stands for Tileset 0.

However the generator doesn't support multiple tilesets.

Perhaps the Tile_IDs should instead be cave_Y0X2 where cave is the tileset name? And then I can implement import of multiple tilesets.

But that would make it possible for Tile_IDs to move around when adding more tilesets. You want people to be able to serialize the Tile IDs and never have the move.

Even with T1X3Y1 it's tricky. What decides what tileset is mapped to T1? How can we be sure that no new tileset ends up before T1?

Perhaps the naming must be like this:

tileset_0_optional_name.ase
tileset_1_optional_name.ase

etc

It will error if something is called tileset_ but not followed by a number.