Adds snippet showing transcription of mensural music to modern (issue 235660043 by address@hidden)

[PhilEHolmes]

Reviewers: Trevor Daniels, J_lowe,

Message:
Please review.

Description:
I've now taken the snippet I added to the LSR concerning using the same
underlying music to create both mensural and modern music, and added it
to the NR where previously there was a ToDo.  It's probably not perfect,
but seems an improvement to me.

Please ignore the headword snippets: this seems a feature of LSR import
and the final push doesn't normally show them.

Please review this at https://codereview.appspot.com/235660043/

Affected files (+569, -478 lines):
  M Documentation/notation/ancient.itely
  M Documentation/snippets/ancient-notation.snippet-list
  M Documentation/snippets/repeats-headword.ly
  M Documentation/snippets/simultaneous-headword.ly
  M Documentation/snippets/text-headword.ly
  A  
Documentation/snippets/using-tags-to-produce-mensural-and-modern-music-from-the-same-source.ly
  M Documentation/snippets/vocal-music.snippet-list