* Previously, Stage was extending UndergroundScene, but
    it needed to be the other way around. Also, the name
    was changed to UndergroundStage.
  * UndergroundStage is the first scene loaded by default.
    Soon, there will be a better way to manage which
    scene is loaded first. For example, the game intro
    would first play, and based on player input a new
    game might be started, which might still first load
    other scenes before the initial UndergroundStage.