This is a button that when pushed pops up a menu (or hierarchy of
menus) defined by an array of
Fl_Menu_Item objects. Motif calls this an OptionButton.
The only difference between this and a Fl_Menu_Button is that the name of the most recent chosen
menu item is displayed inside the box, while the label is displayed
outside the box. However, since the use of this is most often to
control a single variable rather than do individual callbacks, some of
the Fl_Menu_Button methods are redescribed here in those
terms.
When the user picks an item off the menu the value() is set
to that item and then the callback is done.
All three mouse buttons pop up the menu. The Forms behavior of the
first two buttons to increment/decrement the choice is not implemented.
This could be added with a subclass, however.
The menu will also pop up in response to shortcuts indicated by
putting a '&' character in the label(). See Fl_Button for a description of this.
Typing the shortcut() of any of the items will do exactly
the same as when you pick the item with the mouse. The '&' character in
item names are only looked at when the menu is popped up, however.
The value is the index into the Fl_Menu array of the last
item chosen by the user. It is zero initially. You can set it as an
integer, or set it with a pointer to a menu item. The set routines
return non-zero if the new value is different than the old one.
Changing it causes a redraw().
This value is true if the user picks a different value. It is
turned off by value() and just before doing a callback (the
callback can turn it back on if desired).
