Buttons generate callbacks when they are clicked by the user. You
control exactly when and how by changing the values for type()
and when().
The Fl_Light_Button subclass display the "on" state by
turning on a light, rather than drawing pushed in. The shape of the
"light" is initially set to FL_DOWN_BOX. The color of the light when
on is controlled with selection_color(), which defaults to
FL_YELLOW.
