Add virtual int Fl_Group::delete_child(int n) (STR 3218)

This is a convenience method that does range checking (index n),
removes the child given by index n from the group and deletes it.

FL/Fl_Group.H

@@ -1,7 +1,7 @@
 //
 // Group header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2020 by Bill Spitzak and others.
+// Copyright 1998-2021 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -117,6 +117,9 @@ public:
   void remove(Fl_Widget* o) {remove(*o);}
   void clear();
 
+  /* delete child n (by index) */
+  virtual int delete_child(int n);
+
   /**
     Sets the group's resizable widget.
     See void Fl_Group::resizable(Fl_Widget *o)