switching tiles and tile sets

[mlgarrett]

consider implementing a more flexible tile set to allow more color depth.

it would be cool to be able to use whatever (and however many) tiles you wanted from the game. this would ideally include "tiles" (e.g. concrete and landfill), and "entities" (e.g. everything else), which would require some changes to the blueprint construction logic.

it would also be really cool to be able to select tile sets from popular mods.