Prompt user when changing tilesets

[boaromayo]

When opening a different tileset, prompt the user that the map will change based on the tileset about to be loaded.

Then, if user says yes, replace the tiles with the new tileset based on the corresponding IDs on the old tileset.

Meaning if I get a tileset with different dimensions, IDs in that tileset need to be accounted for.

Since every tile is read if a new tileset is loaded on to the panel, this might not be a huge hassle.

One problem, though, might be how to handle different tile sizes. Probably if that happens, change the map grid sizes as well. Note that in the prompt when the user changes tilesets.