
A radio button is similar to a toggle button, with the difference
that only one radio button within the same group can be selected
at any given time. When one radio button is pressed, all other
radio buttons in that group will become unselected.

FLTK provides two ways to create radio buttons. One way is to
set the type of a given button to fltk.FL_RADIO_BUTTON (102).
In this case Fl_Button will behave like Fl_Toggle_Button,
retaining its "down" state when pressed and released.

The second method of creating radio buttons is to use one of
three classes that were omitted from the FLTK documentation:
Fl_Radio_Button is the same as Fl_Button with button type 102
Fl_Radio_Round_Button is <Fl_Round_Button>:type(102)
Fl_Radio_Light_Button is <Fl_Light_Button>:type(102)
