* scripts/musicxml2ly.py (progress): new function

* scripts/musicxml2ly.py (progress): new function
(musicxml_key_to_lily): don't barf on modeless keys.
(create_skip_music): new function.
(musicxml_spanner_to_lily_event): new function. Handle beams too.
(musicxml_note_to_lily_main_event): new function.

* python/musicexp.py (Music.__init__): add comment field.
(NestedMusic.append): new routine.
(SequentialMusic.print_ly): print comment.
(ArpeggioEvent.ly_expression): new class
(BeamEvent.ly_expression): new class
(NoteEvent.__init__): support for cautionary/forced accs.

* lily/lookup.cc (slur): normal order for array loop.