Fix Issue 1035 -- Add context property for negative frets (issue4056041)

[Carl . D . Sorensen]

Reviewers: marc,

Message:
I've posted a patch for fixing issue 1035, by giving the user control
over
what to do with negative fret numbers demanded by an assigned string.

The default behavior is to recalculate the note and put it in the
tablature or fretboard as if it had not had a string assigned.  It can
also be set to ignore the note (leaving it out of the tablature or
fretboard) or to include the note (giving a negative fret number in the
tablature or fretboard.

Please review.

Thanks,

Carl


Description:
Fix Issue 1035 -- Add context property for negative frets
Add handleNegativeFrets, with possibilities of 'ignore,
  'recalculate, 'include

Reorder functions in scm/translation-functions.scm to put
  context in scope of calculate-frets-and-strings

Add regression test.

Please review this at http://codereview.appspot.com/4056041/

Affected files:
  A input/regression/tablature-negative-fret.ly
  M ly/engraver-init.ly
  M scm/define-context-properties.scm
  M scm/translation-functions.scm