Enable different tilesets

[tbeddy]

Currently, only one tileset (from Elite Command) is available. Users should be able to choose from a list of tilesets when starting a new game.

[tbeddy]

@djwhitt Are the tilesets from this GIF complete sets and available somewhere?

[djwhitt]

Those were just showing off the idea and weren't complete tilesets. I'm happy to add other tilesets though as people create them.

[tbeddy]

Since Zetawar could be used for teaching kids, and teachers understandably might not want to use a war game for that, something like a bug/nature tileset would be great to have. Maybe we could commission that later on down the line.