Add support for multi-note tremolo (issue1786041)

[n . puttock]

Hi Reinhold,

This will be a great enhancement if you can sort out the forbidden
breaks between tremolos.

Cheers,
Neil


http://codereview.appspot.com/1786041/diff/1/2
File input/regression/repeat-tremolo-three-notes.ly (right):

http://codereview.appspot.com/1786041/diff/1/2#newcode4
input/regression/repeat-tremolo-three-notes.ly:4: texidoc = "A tremolo
can have more then two notes."
than

http://codereview.appspot.com/1786041/diff/1/2#newcode9
input/regression/repeat-tremolo-three-notes.ly:9: \new Staff \relative
c' {
indent

http://codereview.appspot.com/1786041/diff/1/3
File lily/chord-tremolo-engraver.cc (right):

http://codereview.appspot.com/1786041/diff/1/3#newcode60
lily/chord-tremolo-engraver.cc:60: Grob *previous_grob_;
Since it's always a stem, why not call it previous_stem_ or last_stem_?

http://codereview.appspot.com/1786041/diff/1/3#newcode87
lily/chord-tremolo-engraver.cc:87: announce_end_grob (beam_,
previous_grob_->self_scm ());
This is too late to be acknowledged by the Spanner_break_forbid_engraver
before the next timestep, which means breaks are always forbidden
between consecutive tremolos.

http://codereview.appspot.com/1786041/diff/1/3#newcode130
lily/chord-tremolo-engraver.cc:130: Stem::set_beaming (previous_grob_,
flags, RIGHT);
mixing tabs and spaces

http://codereview.appspot.com/1786041/show