Re: Various clean-ups in stems and beams. (issue 6584045)

[mtsolo]

Reviewers: aleksandr.andreev,

Message:
I think a user should be able to use Kievan notation and normal
stems/flags/beams if she so chooses. I'll define something like

startKievanNotation = {
   %% Set glyph styles.
 \override NoteHead #'style = #'kievan
 \override Rest #'style = #'mensural
 \override Accidental #'glyph-name-alist =
#alteration-kievan-glyph-name-alist
 \override Dots #'style = #'kievan
 \override Slur #'stencil = ##f

 %% There are beams in Kievan notation, but they are invoked manually
 \set autoBeaming = ##f
}

stopKievanNotation = {
 \revert NoteHead #'style
 \revert Rest #'style
 \revert Accidental #'glyph-name-alist
 \revert Dots #'style
 \revert Slur #'stencil

 \unset autoBeaming

}

and then use it in the documentation.

Description:
Various clean-ups in stems and beams.

*) Eliminates code dups for Kievan work.
*) Transfers functions that are called a lot to C++ to speed things up.
*) Eliminates unused variables.

No regtest changes.

Please review this at https://codereview.appspot.com/6584045/

Affected files:
  A input/regression/kievan-notation.ly
  M lily/include/beam.hh
  M lily/include/stem.hh
  M lily/stem.cc
  M ly/engraver-init.ly
  M scm/define-grobs.scm
  M scm/output-lib.scm