More State class changes. All state functionality has been moved from the Game class to State.

* Game.cs:
  Remove state modifiers.

* Program.cs:
  Converted Game.GetState call to State.Get.

* GameState.cs, MenuState.cs, EndGameState.cs, DifficultyState.cs:
  Converted to use new state methods.

* State.cs:
  Moved state modifiers from the Game class to here.
  Removed UpdateChanged(), DrawChanged(), and StatusChanged() and
  replaced them with Enter() and Leave(), which are called when
  Updating is set.