add choral and choral-cautionary accidental style (issue 311430043 by address@hidden)

[perpeduumimmobile]

Reviewers: ,

Message:
add choral and choral-cautionary accidental style

This solves the issues mentioned in
  https://lists.gnu.org/archive/html/lilypond-user/2016-12/msg00201.html
that accidental style could not apply to ChoirStaves in the same way as
they did for PianoStaves, StaffGroups or GrandStaves. No impact on
existing functionality, but now ChoirStaves are equal siblings w.r.t.
accidental styles.

In addition, a reasonable mix of the modern-voice and piano cautionary
style is defined, which caters to both all-observant and egocentric
readers.

Description:
add choral and choral-cautionary accidental style

This solves the issues mentioned in
  https://lists.gnu.org/archive/html/lilypond-user/2016-12/msg00201.html
that accidental style could not apply to ChoirStaves in the same way
as they did for PianoStaves, StaffGroups or GrandStaves.
No impact on existing functionality, but now ChoirStaves are equal
siblings w.r.t. accidental styles.

In addition, a reasonable mix of the modern-voice and piano cautionary
style is defined, which caters to both all-observant and egocentric
readers.

Please review this at https://codereview.appspot.com/311430043/

Affected files (+151, -1 lines):
  M Documentation/notation/pitches.itely
  M ly/engraver-init.ly
  M scm/music-functions.scm