Added difficulty selection.

* Resource.cs:
  Moved.

* DifficultyState.cs:
  Initial version.

* Game.cs:
  Added difficulty selection state.

* GameState.cs, BlockSpawner.cs:
  Updated to use difficulty settings.

* MenuState.cs:
  Switched to use OnEndFrame when enter is pressed.

* LinuxUtil.cs:
  Cleanup.