Uncovered latent bugs

I've implemented my 'restart' & quit level menu buttons and uncovered a couple of bugs as result:

1) When switching screens, there is a potential bug in the frameworks that I patterned after Mario Zechner's game screen implementation.  Two methods are executed from the libgdx's render() method: update() and then present() -- in that order.  If the update() method logic decides to switch screens, the Game instance will do just that... and then run the new screen's present() method immediately upon returning to the render() method.  This could be a problem if the present() method is making the assumption that the update() has executed at least once.  I added a transition check in the Game class to ensure that a guaranteed transition will occur at the end of the Game's render() method.  The alternatives are to: ignore this behavior (most screens probably don't care) or encode the actual transition in the Screen's present() method.

2) My Level class was braindead.  Some levels included push buttons, doors, and wire lists that are instantiated when the Level class is instantiated.  When I reset the level, the lists were updated without being destroyed before hand.  Oops.  The net result was that after several instances of resetting the level, I'd get a native code error while trying to look up a prismatic joint translation.  In essence, the lists were maintaining references to dead objects (my world object is destroyed and recreated at every reset).  Yeah, that's classically what is known as a "BAD IDEA (tm)".  At any rate.  FIXED.  I added a dispose method to the parent Level class to remind me that I need to nuke those objects on level exit / reset.