Clean up inconsistencies in engraver-init.ly and performer-init.ly

Clean up inconsistencies in engraver-init.ly and performer-init.ly

issue 4281

In performer-init.ly
- let context-defs be copied when possible
- add missing contexts for \accepts and \alias
- delete settings for ChordNameVoice
- add GregorianTranscriptionStaff, GregorianTranscriptionVoice, NoteNames,
  PetrucciStaff, PetrucciVoice
- all contexts are written without quotation marks
- accepted contexts are ordered alphabetically

In engraver-init.ly
- add FretBoards to the accepted contexts in StaffGroup
- add NullVoice to the accepted contexts in RhythmicStaff
- all contexts are written with quotation marks
- accepted contexts are ordered alphabetically