Made the Paddle class only render, updating with 'controllers' is now done in the PhysicsWorld with the implementation's consent, meaning it might not happen.

 - Updated the SinglePlayerGameState to use this new system.
 - The SimplePhysicsWorld though, has been basically downgraded. It doesn't move the ball, only updates the paddles using their respective controllers. No more, no less.
Moved the BallWrapper and PaddleWrapper classes into the protected scope of the PhysicsWorld class.

That's because the two classes are more of an implementation detail then anything else.