Refactor get/set_property to take the item as first argument

lily/balloon-engraver.cc

 /*
   This file is part of LilyPond, the GNU music typesetter.
 
   Copyright (C) 2006--2020 Han-Wen Nienhuys <hanwen@lilypond.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 42 matching lines @@
 
 Balloon_engraver::Balloon_engraver (Context *c)
   : Engraver (c)
 {
 }
 
 void
 Balloon_engraver::balloonify (Grob *g, Stream_event *event)
 {
   Grob *b = make_item ("BalloonTextItem", event->self_scm ());
-  b->set_property ("text", event->get_property ("text"));
+  set_property (b, "text", get_property (event, "text"));
   b->set_parent (g, Y_AXIS);
   b->set_parent (g, X_AXIS);
 }
 
 void
 Balloon_engraver::acknowledge_grob (Grob_info info)
 {
   Stream_event *cause = info.event_cause ();
 
-  SCM arts = cause ? cause->get_property ("articulations") : SCM_EOL;
+  SCM arts = cause ? get_property (cause, "articulations") : SCM_EOL;
   for (SCM s = arts; scm_is_pair (s); s = scm_cdr (s))
     {
       Stream_event *e = unsmob<Stream_event> (scm_car (s));
       if (e->in_event_class ("annotate-output-event"))
         {
           balloonify (info.grob (), e);
         }
     }
 
   for (vsize i = 0; i < events_.size (); i++)
     {
-      if (info.grob ()->name () == ly_symbol2string (events_[i]->get_property ("symbol")))
+      if (info.grob ()->name () == ly_symbol2string (get_property (events_[i], "symbol")))
         balloonify (info.grob (), events_[i]);
     }
 }
 
 void
 Balloon_engraver::boot ()
 {
   ADD_LISTENER (Balloon_engraver, annotate_output);
   ADD_ACKNOWLEDGER (Balloon_engraver, grob);
 }
 
 ADD_TRANSLATOR (Balloon_engraver,
                 /* doc */
                 "Create balloon texts.",
 
                 /* create */
                 "BalloonTextItem ",
 
                 /*read*/
                 "",
 
                 /*write*/
                 ""
                );