Support slurs starting and/or ending on individual notes in chords

Support slurs starting and/or ending on individual notes in chords

Sorry, I can't actually come up with satisfactory examples right now.
Please find something in Chopin or so.  I'm sure we discussed this on
the mailing list at some point of time.


Also contains commits:

Accept note heads as slur bounds

Also changes base_attachment calculation for in-chord slurs.


Move in-chord slur info to slur boundaries


Listen to and record in-chord slurs


Add event_symbol function to all slur engravers


Rename Slur_proto_engraver::internal_listen_slur to listen_slur

That's more in line with how we divert to base class functions elsewhere.