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_Check_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_DIAMOND_DOWN_BOX. The color of the
light when on is controlled with selection_color(), which
defaults to FL_RED.