Refactor get/set_property to take the item as first argument

lily/spacing-engraver.cc

 /*
   This file is part of LilyPond, the GNU music typesetter.
 
   Copyright (C) 1999--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 184 matching lines @@
 void
 Spacing_engraver::stop_translation_timestep ()
 {
   Paper_column *musical_column
     = unsmob<Paper_column> (get_property (this, "currentMusicalColumn"));
 
   if (!spacing_)
     start_spanner ();
 
   set_object (musical_column, "spacing", spacing_->self_scm ());
-  unsmob<Grob> (get_property (this, "currentCommandColumn"))
-  ->set_object (this, "spacing", spacing_->self_scm ());
+  set_object (unsmob<Grob> (get_property (this, "currentCommandColumn")),
+              "spacing", spacing_->self_scm ());
 
   SCM proportional = get_property (this, "proportionalNotationDuration");
   if (unsmob<Moment> (proportional))
     {
       set_property (musical_column, "shortest-playing-duration", proportional);
       set_property (musical_column, "shortest-starter-duration", proportional);
       set_property (musical_column, "used", SCM_BOOL_T);
       return;
     }
 
@@ skipping 65 matching lines @@
                 "SpacingSpanner ",
 
                 /* read */
                 "currentMusicalColumn "
                 "currentCommandColumn "
                 "proportionalNotationDuration ",
 
                 /* write */
                 ""
                );