fix handling of grace notes to shorten preceding tied notes correctly (issue 7108043)

[k-ohara5a5a]

Congratulations on finding the cause of the problem, through the maze of
objects implementing MIDI output.


https://codereview.appspot.com/7108043/diff/1/input/regression/midi-grace-after-tie.ly
File input/regression/midi-grace-after-tie.ly (right):

https://codereview.appspot.com/7108043/diff/1/input/regression/midi-grace-after-tie.ly#newcode3
input/regression/midi-grace-after-tie.ly:3: texidoc = "Grace notes don't
break ties which immediately precede them."
Tied notes sound as one note in MIDI.  Grace notes following a tied note
shorten the resulting single note in MIDI.

https://codereview.appspot.com/7108043/diff/1/lily/include/audio-item.hh
File lily/include/audio-item.hh (right):

https://codereview.appspot.com/7108043/diff/1/lily/include/audio-item.hh#newcode90
lily/include/audio-item.hh:90: virtual string to_string () const;
It seems fine to have the debug-output helper to_string(), but why make
it a virtual member function?  Unless the compiler is a lot smarter than
me, that would seem to require a pointer to the actual function be
stored in every Audio_note object.  Then every user would need extra
memory for every note, but would get no benefit.

https://codereview.appspot.com/7108043/