Modify fret calculation algorithm (issue3320041)

[Carl . D . Sorensen]

Reviewers: steve yegge <steve.yegge_gmail.com>, patrick schmidt  
<p.l.schmidt_gmx.de>,

Message:
Here's a patch to the fret calculator that answers the first two of
Steve's requests, I think.

http://article.gmane.org/gmane.comp.gnu.lilypond.general/60177

It turns a -0 fingering into an open string request, and it creates a
context property defaultStrings that will serve as a request for strings
to be used if the chord doesn't include specific string indications.

Steve and Patrick, you can only test this if you can build the code,
because this includes C++ changes.

Please review.

Thanks,

Carl


Description:
Modify fret calculation algorithm
Zero fingering creates an open string fret.

Add defaultStrings property to define strings that will be used
 in calculating frets if no string number is specified in a chord.

Add regression tests for both of these cases.

Please review this at http://codereview.appspot.com/3320041/

Affected files:
  A input/regression/tablature-default-strings.ly
  A input/regression/tablature-zero-finger.ly
  M lily/fretboard-engraver.cc
  M lily/tab-note-heads-engraver.cc
  M scm/define-context-properties.scm
  M scm/translation-functions.scm