Adds Screen2D.worldCenter() and Screen2D.worldCenterAtZ().
Adds two more levels, making five total.
Fixes bugs, minor refactor.

- Refactors to use Screen2D.worldCenterAtZ().
- Moves current stage number out of mission object and into
  LevelManager. Mission objects shouldn't be modified after load.
  This fixes missions only having 3 stages after a previously-failed
  game.
- Fixes duplicate game board object creation due to race condition
  when calling LevelManager.resetBoard() twice after a failed game.