Move input logic to own module
Implement selectors for the players

Worldstate.hs
* gameActivePlayer returns the player that is supposed to move
* gameOtherPlayer returns the player that has to wait for their turn
Implement Renderable class

Board.hs:
* Board is now an instance of renderable
Output.hs
* Create a class for output that implements a render function to put
  stuff to the screen via the IO Monade
Make Game an instance of Renderable
Reimplement game main loop

We use the modules we prepared in the last commits to run the game.