Multiple states can now be displayed and / or updated at the same time.
Note: a cleaner version will be the next "major" goal.

* AIdleState.cs:
  Initial version.

* Game.cs:
  Added SetStateStatus.

* DifficultyState.cs:
  Converted to AIdleState.
  Cleanup.

* EndGameState.cs:
  Converted to AIdleState.

* BlockSpawner.cs:
  Usage change.

* Player.cs:
  Added Reset() method.

* GameState.cs:
  Cleanup.
  Changed for usage of multiple states.

* MenuState.cs:
  Added resume button when in-game.
  Cleanup.