Refactor get/set_property to take the item as first argument

lily/rest-engraver.cc

 /*
   This file is part of LilyPond, the GNU music typesetter.
 
   Copyright (C) 1997--2020 Han-Wen Nienhuys <hanwen@xs4all.nl>
 
   LilyPond is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.
 
@@ skipping 49 matching lines @@
   rest_ = 0;
   dot_ = 0;
 }
 
 void
 Rest_engraver::process_music ()
 {
   if (rest_event_ && !rest_)
     {
       rest_ = make_item ("Rest", rest_event_->self_scm ());
-      Pitch *p = unsmob<Pitch> (rest_event_->get_property ("pitch"));
+      Pitch *p = unsmob<Pitch> (get_property (rest_event_, "pitch"));
 
       if (p)
         {
           int pos = p->steps ();
-          SCM c0 = get_property ("middleCPosition");
+          SCM c0 = get_property (this, "middleCPosition");
           if (scm_is_number (c0))
             pos += scm_to_int (c0);
 
-          rest_->set_property ("staff-position", scm_from_int (pos));
+          set_property (rest_, "staff-position", scm_from_int (pos));
         }
     }
 }
 
 void
 Rest_engraver::listen_rest (Stream_event *ev)
 {
   ASSIGN_EVENT_ONCE (rest_event_, ev);
 }
 
 void
 Rest_engraver::boot ()
 {
   ADD_LISTENER (Rest_engraver, rest);
 }
 
 ADD_TRANSLATOR (Rest_engraver,
                 /* doc */
                 "Engrave rests.",
 
                 /* create */
                 "Rest ",
 
                 /* read */
                 "middleCPosition ",
 
                 /* write */
                 ""
                );