Fix for string-number (and other) articulations in chords and tablature

[Carl Sorensen]

Hi,

A couple of months ago I submitted a patch to make the
tab-note-heads-engraver and the fretboards-engraver use the same code to
convert from notes (with string-number articulations/events) to
tablature/fretboard entries.

There were two undesirable side effects of that patch: 1) We had duplicated
code in the tab-note-heads-engraver and the fretboards-engraver; and 2) the
code as written didn't work if only some of the notes had string numbers
identified.

I've posted a patch on Rietveld that solves both of these issues.  The
common code that matches up notes and string-number events/articulations has
moved to a new file, lily/articulations.cc, where it is called by both
tab-note-heads-engraver and fretboards-engraver.  Also, I fixed the code so
that it recognizes null string numbers, so partial string numbering will be
appropriately recognized.

I'm not sure that I've handled things appropriately, although everything
works.

Please review the patch; I'd welcome any comments you might have.

<http://codereview.appspot.com/906045>

Thanks,

Carl