Default Box2DWorld in the scenetree

[briansemrau]

There should be a default Box2DWorld singleton so that scenes do not need to add a Box2DWorld to be able to simulate.

Note:
This will change the way the current b2 object lifecycle works. Rather than creating/destroying only in ENTER/EXIT_TREE, b2 objects will be created on READY or in the constructor (whichever Godot uses in their physics nodes). This may leave some optimization to be desired, as creating a new body in a non-default space might always follow the path: [READY: create in default world] -> [ENTER: delete from default, create in new space from tree]