Moved most game logic into World class, and added preliminary keyboard interaction to player's paddle
Added some boundary checking so the paddle can't go off the screen
Added stationary Birdie. Next up, physics
Added some realistic physics to the Birdie. It now bounces off the walls in a realistic manner. Had to make some adjustments to the structure to accomodate a reasonable speed, and modified the World object to update Birdie movement outside the event loop