Tweaked 2d Physics body creation, exposed more Box2D features to the engine.

- Creation of physics bodies now respect any compound tranformations of
a ScreenEntity

- PhysicsScreenEntity can now change angular and linear damping,
friction, density, group index, and collision category and masks

- Collision-only PhysicsScreenEntities can optionally be set as not a
sensor, so contact information is preserved in the manifold (sensors
do not provide collision point or normals in collision events). Non-sensor
collisions are still disabled before they hit the solver.