Added dungeon rooms finisher.

This implements #1260.

Only partially done, do not merge yet.

The DungeonRooms finisher will add dungeon rooms to the generated worlds. The rooms' placement is based on a GridStructGen, the rooms will contain 2 chests each (rarely 1 chest only).

TODO:
- [ ] Fix chests placement when forming a double-chest
- [ ] Improve chest placement - select chest coords dependently to avoid chest overwriting.
- [ ] Place a random mob spawner in the middle of the room
- [ ] Make the floor a random pattern of cobblestone / mossy cobblestone
The only reason was me not remembering. Fixing now.