model the fixed, animated toolbar/navbar in physics

[urbien]

Right we do not add a body corresponding to navbar to the world (physics engine's view of the UI). That means we also can not do cool things with it, move it, bump it, morph it, hide it, unhide it with the natural physic-y animations. But when we add navbar to the world, we will need to rethink how scroll bounces off of it.