This widget simply draws its box, and possibly it's label. Putting it
before some other widgets and making it big enough to surround them
will let you draw a frame around them.
The first constructor sets box() to FL_NO_BOX, which
means it is invisible. However such widgets are useful as placeholders
or Fl_Group::resizable()
values. To change the box to something visible, use box(n).
The second form of the constructor sets the box to the specified box
type.