A Ball is an abstract class that does nothing -- it is just a convenient name
for a "generic" ball, that is, any Drawable, Relocatable, Animate object.
Associates a type of Ball and a BallEnvironment with this button,
labels this button with the type of Ball, and arranges
for this button to respond to its own button-presses.
Adds the Bouncer to its World at the middle of the World
with the default color and diameter for a Bouncer,
and with a constant but randomly chosen velocity.