Automatically handle adding a world to a Scene if physics is set to
true.  This also adds world as a property to Scene.

This means we no longer need to include a World inside the scene, for
entities to participate in the world, just need physics = true.
Wrap signals and properties from Box2DWorld with Scene
Merge branch 'world-in-scene' of github.com:kenvandine/Bacon2D into world-in-scene