prevent ball falling through world

I think this issue was raised in #22
but it is also obvious when playing level_full: when you get too far from the golf ball, it falls through the world.
I re-parented the ball to the scene (rather than the buggy) to prevent this from happening. As far as I can tell, I hooked everything back up so that it all works the same as before the re-parent (except, it no longer falls through the world)