Refactor get/set_property to take the item as first argument

lily/rhythmic-music-iterator.cc

 /*
   This file is part of LilyPond, the GNU music typesetter.
 
   Copyright (C) 2012--2020 Mike Solomon <mike@mikesolomon.org>
 
   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 30 matching lines @@
 void
 Rhythmic_music_iterator::process (Moment m)
 {
   if (last_processed_mom_ < Moment (0))
     {
 
       descend_to_bottom_context ();
 
       Context *c = get_outlet ();
       Stream_event *ev = get_music ()->to_event ();
-      SCM arts = ev->get_property ("articulations");
+      SCM arts = get_property (ev, "articulations");
 
       if (scm_is_pair (arts))
         {
           // There is no point in broadcasting articulations like
           // harmonic events that nobody listens to.  Those work
           // exclusively as articulations.
           SCM listened = SCM_EOL;
           SCM unlistened = SCM_EOL;
           for (; scm_is_pair (arts); arts = scm_cdr (arts))
             {
               SCM art = scm_car (arts);
 
               if (c->event_source ()->is_listened_class
-                  (unsmob<Stream_event> (art)->get_property ("class")))
+                  (get_property (unsmob<Stream_event> (art), "class")))
                 listened = scm_cons (art, listened);
               else
                 unlistened = scm_cons (art, unlistened);
             }
-          ev->set_property ("articulations", scm_reverse_x (unlistened, SCM_EOL));
+          set_property (ev, "articulations", scm_reverse_x (unlistened, SCM_EOL));
           c->event_source ()->broadcast (ev);
           arts = scm_reverse_x (listened, SCM_EOL);
           for (; scm_is_pair (arts); arts = scm_cdr (arts))
             c->event_source ()->broadcast (unsmob<Stream_event> (scm_car (arts)));
         }
       else
         c->event_source ()->broadcast (ev);
 
       ev->unprotect ();
     }
   Simple_music_iterator::process (m);
 }
 
 IMPLEMENT_CTOR_CALLBACK (Rhythmic_music_iterator);