Add animation for the tipping of the balance beam

[jbphet]

@amanda-phet, @marlitas, and I met this morn to discuss a few design aspects for this sim, and we decided to try animating some situations related to the balance beam in the 3rd screen. Animation would be used when the supports are removed and potentially used when a new ball is kicked onto the field. Animation will not be used when the movable fulcrum is being adjusted by the user, since the changes are small and quick and it almost looks animated already.

[jbphet]

@amanda-phet - I have an implementation of this working. I could describe a bunch of things about how it works and why, but I think our best move is to have you take it for a test drive and let @marlitas and I know what you think.

[amanda-phet]

This is really nice! I like it a lot. I'll go ahead and close since I don't have any change requests.