Added three buttons to the menu.

 - Singleplayer
   - Does not do anything as of now.
 - Multiplayer
   - Exits the game as of now.
 - Quit
   - Also exits the game.

Each button is properly placed.
Added the start of a single player game state. It is nonfunctional, but the MenuGameState still switchs to it when the user clicks the "Singleplayer" button.