Add option to merge tilesets from different layers into just one tileset

[dacap]

If we have tilemap layers with similar tilesets, we might want to merge all their tilesets into just one (remapping the tile references of each tilemap).

We could provide an option to do this in the Sprite > Properties dialog as a quick solution in some way, or create a new "Tileset Management" dialog in some way (I'll create another issue for this).