quarter tones in tablature (issue 272320043 by address@hidden)

[thomasmorley65]

Reviewers: ,

Message:
Please review

Description:
quarter tones in tablature

issue 4643

micro-tones in TabStaff are now printable.  Unless the chosen
string-tuning will allow it, this feature is disabled for
FretBoards.
- changing determine-frets (adding an optional argument) and
  fret-number-tablature-format
- adding a regtest with quarter-tone-string-tuning
- documenting it in Documentation/notation/fretted-strings.itely
  and Documentation/changes.tely

Please review this at https://codereview.appspot.com/272320043/

Affected files (+129, -8 lines):
  M Documentation/changes.tely
  M Documentation/notation/fretted-strings.itely
  A input/regression/tablature-micro-tone.ly
  M ly/engraver-init.ly
  M scm/translation-functions.scm