Stuff left:
    1) Add sound support to the BaseState subclasses.
    2) Implement the NPC spawners.
    3) Calculate socres and wave increases.
    4) Implement the defeat condition with the enemies.
    5) Center the letters in ScoreState and the story labels in MenuState.
    6) Fix the issue of the right wall not properly aligned on Android.
    7) Make movement relative to player position.
    8) Make npcs change direction randomly.
    9) Make npcs run away from player
