Added balloon test, refactored GameManager and added SceneManager

Playing around with balloons in 06_BalloonTest

Also refactored a bunch of things that were communicating directly with
one another, and moved them to NotificationCenter.

Added a GameState class to help GameManager with pausing as well, and
added finer grain controls to the character states (eg. CanPounce,
CanFall, CanMove, CanSing)

Also fixed a few little issues in the menu