Added draw ordering and made the menu and difficulty selection states transparent.

* ForegroundState.cs:
  Initial version.

* Player.cs:
  Added WallAllowance variable.

* Game.cs:
  Added shortcut methods to allow for multiple states.
  Added draw sorting.

* DifficultyState.cs:
  Converted to ForegroundState.
  Now leaves the GameState drawing when returning to the menu.

* GameState.cs:
  Added escape key handling.
  Now leaves the state drawing when going to end_game.

* MenuState.cs:
  Converted to ForegroundState.
  Now leaves the game state drawing when going to difficulty_select.

* State.cs:
  Made _updating and _drawing protected.
  Added static variable MaxDrawOrder.
  Added DrawOrder.