Adding a light source

Took a first step towards implementing shadowcasting, the ability to show a tile as lit/unlit/hidden.

Very first implementation is just distance based. Next step: recursive shadowcasting! =D

To be totally honest, this setup of the generated dungeon doesn’t feel very interesting to explore. But that’s not my main concern right now. I’m just trying to get the very basic systems to work before moving on with the gameplay parts.