Fix for "Fl_Pack should override clear() to set resizable(0)" (#993)

FL/Fl_Pack.H

@@ -93,6 +93,9 @@ public:
   uchar horizontal() const {return type();}
 
   void resize(int X, int Y, int W, int H) FL_OVERRIDE;
+  /** Deletes all child widgets with Fl_Group::clear().
+  And  sets to NULL the resizable() widget. */
+  void clear() { Fl_Group::clear(); resizable(NULL); }
 };
 
 #endif