Let the distance of strings and frets in fret-diagrams be settable (issue 319030043 by address@hidden)

[thomasmorley65]

Reviewers: ,

Message:
Please review

Description:
Let the distance of strings and frets in fret-diagrams be settable

- Two new sub-properties of fret-diagram-details are introduced:
  fret-distance and string-distance
- regtest fret-diagrams-size.ly is extended
- entry in changes.tely

Please review this at https://codereview.appspot.com/319030043/

Affected files (+116, -36 lines):
  M Documentation/changes.tely
  M input/regression/fret-diagrams-size.ly
  M scm/define-grob-properties.scm
  M scm/fret-diagrams.scm