Merge branch 'master' into lilypond/translation

* master:
  Add 'See also's to LM 3 'Fundamental concepts'
  Move sections in French and German docs
  GDP LM 3.3.4 Explaining \context
  Updates to chords.itely, fretted-strings.itely
  Compile fix.
  Update from Kurt.
  Update fretted-strings.itely
  staff-symbol-referencer.cc: Junk redundant functions.
  Robustify Staff_symbol_referencer::on_line ().
  Update fretted-strings.itely
  Change type predicate for 'layer to integer.
  Replacing "User Manual" with "Notation Ref" in SL