SubSystems are mini Indigo games. They contain similar functions to the full games, and allow you to
compartmentalise parts of your game logic. They are strictly separated from the rest of your game, and can only
communicate with your game via events. They hold their own state, and rely on the composable nature of
SceneUpdateFragments to (optionally) present game elements back to the player. Sub systems can be used for all
sorts of things, such as rendering parallax backgrounds or encapsulating communication with a browser / network.