Refactor get/set_property to take the item as first argument

lily/fretboard-engraver.cc

 /*
   This file is part of LilyPond, the GNU music typesetter.
 
   Copyright (C) 2006--2020 Han-Wen Nienhuys
 
   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 84 matching lines @@
     return;
 
   SCM tab_strings = articulation_list (note_events_,
                                        tabstring_events_,
                                        "string-number-event");
   SCM fingers = articulation_list (note_events_,
                                    fingering_events_,
                                    "fingering-event");
   fret_board_ = make_item ("FretBoard", note_events_[0]->self_scm ());
   SCM fret_notes = ly_cxx_vector_to_list (note_events_);
-  SCM proc = get_property ("noteToFretFunction");
+  SCM proc = get_property (this, "noteToFretFunction");
   if (ly_is_procedure (proc))
     scm_call_4 (proc,
                 context ()->self_scm (),
                 fret_notes,
                 scm_list_2 (tab_strings, fingers),
                 fret_board_->self_scm ());
-  SCM changes = get_property ("chordChanges");
-  SCM placements = fret_board_->get_property ("dot-placement-list");
+  SCM changes = get_property (this, "chordChanges");
+  SCM placements = get_property (fret_board_, "dot-placement-list");
   if (to_boolean (changes)
       && ly_is_equal (last_placements_, placements))
-    fret_board_->set_property ("begin-of-line-visible", SCM_BOOL_T);
+    set_property (fret_board_, "begin-of-line-visible", SCM_BOOL_T);
 
   last_placements_ = placements;
 }
 
 void
 Fretboard_engraver::stop_translation_timestep ()
 {
   fret_board_ = 0;
   note_events_.clear ();
   tabstring_events_.clear ();
@@ skipping 24 matching lines @@
                 "minimumFret "
                 "noteToFretFunction "
                 "predefinedDiagramTable "
                 "stringTunings "
                 "tablatureFormat ",
 
                 /* write */
                 ""
                );