Refactor event_listener.py

[ChrisNeedham24]

Yes, I know it was only just made in #150 but it could definitely be improved. The main two ways I can see this going are to split RequestHandler and EventListener into their own files, and potentially going even further, to split a sort of UpdateHandler out of RequestHandler.

[ChrisNeedham24]

Could also get even more tricky and extract game state modification out of both EventListener and GameState into some common file, but that would understandably be quite difficult.