Refactor the LevelManager into a module like textures #59

Currently I have the level management and the current game level everywhere, which makes sense because it's a core thing in the game. Based on that it'd be better if LevelManager was a module, System just accessed it directly, and nothing else had a reference to the current level. Anything that needs the current level goes through the levelmanager module, or uses System.

That could also be the protocol: Nothing actually has the current level, levelmanager is basically a database, and Sytem is in charge of managing it.