Automatically stop quacking when sound is finished

Set up a timer to call stopQuacking() after the quackSound has finished
playing

This doesn't trigger an error from the world being locked because it is
done on the same thread
Allow subclasses of Entity to set sprite origin

Needed to center the bodies of Player, which doesn't use the super
constructor that previously dealt with centering a body in its sprite
Extract constant for player body size and center a smaller body