Issue 4478: Disentangle some Tie and Semi_tie code. (issue 251840044 by address@hidden)

[nine . fierce . ballads]

Reviewers: ,

Description:
Both are Grobs, but ties are Spanners and semi-ties are Items.  The code
for both cases was jumbled together in many Tie methods.  I hope that I
have clarified a few things.  No functional changes are intended.

Please review this at https://codereview.appspot.com/251840044/

Affected files (+111, -87 lines):
  M lily/completion-note-heads-engraver.cc
  M lily/include/semi-tie.hh
  M lily/include/tie.hh
  M lily/include/tie-column.hh
  M lily/semi-tie.cc
  M lily/tie.cc
  M lily/tie-column.cc
  M lily/tie-engraver.cc
  M lily/tie-formatting-problem.cc
  M lily/tie-specification.cc