Fix removal and deletion of the resizable() of Fl_Group

If the resizable() of an Fl_Group is deleted it is automatically
removed from the group.

New: to avoid dangling resizable() pointers the resizable widget
is set to the group itself.

FL/Fl_Group.H

@@ -1,7 +1,7 @@
 //
 // Group header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2021 by Bill Spitzak and others.
+// Copyright 1998-2022 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
@@ -152,6 +152,14 @@ public:
     dimensions of the box are scaled to the new size. Widgets outside the
     box are moved.
 
+    \note The resizable of a group \b must be one of
+      - NULL
+      - the group itself
+      - a direct child of the group.
+    \note If you set any other widget that is not a direct child of the group as
+      its resizable then the behavior is undefined. This is \b not checked by
+      Fl_Group for historical reasons.
+
     In these examples the gray area is the resizable:
 
     \image html resizebox1.png

src/Fl_Group.cxx

@@ -501,6 +501,7 @@ void Fl_Group::remove(int index) {
   if (index < 0 || index >= children_) return;
   Fl_Widget &o = *child(index);
   if (&o == savedfocus_) savedfocus_ = 0;
+  if (&o == resizable_) resizable_ = this;
   if (o.parent_ == this) {      // this should always be true
     o.parent_ = 0;
   }