The Fl_Multi_Browser class is a subclass of Fl_Browser
which lets the user select any set of the lines. The user interface
is Macintosh style: clicking an item turns off all the others and
selects that one, dragging selects all the items the mouse moves over,
and shift + click toggles the items. This is different then how forms
did it. Normally the callback is done when the user releases the
mouse, but you can change this with when().
See Fl_Browser for
methods to add and remove lines from the browser.