Issue 4427: clean up finding & changing of contexts in C++ (issue 238670043 by address@hidden)

[nine . fierce . ballads]

Reviewers: ,

Description:
clean up finding & changing of contexts in C++

Combine the key feature of Change_iterator and Auto_change_iterator
into a common function (favoring the logic of Change_iterator).

Define functions for various context-search loops.

Please review this at https://codereview.appspot.com/238670043/

Affected files (+128, -146 lines):
  M lily/accidental-engraver.cc
  M lily/auto-change-iterator.cc
  M lily/change-iterator.cc
  M lily/context.cc
  M lily/grace-engraver.cc
  M lily/include/change-iterator.hh
  M lily/include/context.hh
  M lily/lyric-combine-music-iterator.cc
  M lily/lyric-engraver.cc
  M lily/note-head-line-engraver.cc