Refactor get/set_property to take the item as first argument

lily/cluster.cc

 /*
   This file is part of LilyPond, the GNU music typesetter.
 
   Copyright (C) 2002--2020 Juergen Reuter <reuter@ipd.uka.de>
   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 22 matching lines @@
 using std::vector;
 
 /*
   TODO: Add support for cubic spline segments.
  */
 Stencil
 brew_cluster_piece (Grob *me, vector<Offset> bottom_points, vector<Offset> top_points)
 {
   Real blotdiameter = Staff_symbol_referencer::staff_space (me) / 2;
 
-  Real padding = robust_scm2double (me->get_property ("padding"), 0.0);
+  Real padding = robust_scm2double (get_property (me, "padding"), 0.0);
 
   Offset vpadding = Offset (0, padding);
   Offset hpadding = Offset (0.5 * blotdiameter, 0);
   Offset hvpadding = 0.5 * hpadding + vpadding;
 
-  SCM shape_scm = me->get_property ("style");
+  SCM shape_scm = get_property (me, "style");
   string shape;
 
   if (scm_is_symbol (shape_scm))
     shape = ly_symbol2string (shape_scm);
   else
     {
       programming_error ("ClusterSpanner.style should be defined as a symbol.");
       me->suicide ();
       return Stencil ();
     }
@@ skipping 176 matching lines @@
 {
 public:
   DECLARE_SCHEME_CALLBACK (height, (SCM));
 };
 
 MAKE_SCHEME_CALLBACK (Cluster_beacon, height, 1);
 SCM
 Cluster_beacon::height (SCM g)
 {
   Grob *me = unsmob<Grob> (g);
-  Interval v = robust_scm2interval (me->get_property ("positions"),
+  Interval v = robust_scm2interval (get_property (me, "positions"),
                                     Interval (0, 0));
   return ly_interval2scm (Staff_symbol_referencer::staff_space (me) * 0.5 * v);
 }
 
 ADD_INTERFACE (Cluster_beacon,
                "A place holder for the cluster spanner to determine the"
                " vertical extents of a cluster spanner at this"
                " X@tie{}position.",
 
                /* properties */
                "positions "
               );