Sketch for in-notes. (issue 5293053)

[mtsolo]

Reviewers: ,

Message:
Hey all,

As part of project markup & at the suggestion of Nicolas Sceaux by way
of Valentin's opera, I've implemented in-notes in LilyPond.  These notes
hover above systems and come up a fair bit in vocal music.

It works, but I have one major and one semi-major snag that I'd like to
run by people.  Please don't run the regtests on this yet until these
snags are worked out.

Semi-major-snag: get_footnotes_from_lines should only ever be called
once and should stash its results in the systems and/or probs.  I'll
figure out how to do this in the near future.

Major snag.  On about 1 out of every 5 runs of this patch, it leads to
way-too-big spacing.  This is because the stencils of in notes are, for
some reason, registering infinite extents in page-spacing.cc when
they're being counted (check account_for_footnotes and you'll see that
the value of the extent for in_notes is occasionally really really
really large).  This is strange because when the in_notes are added to
the breaker (System::get_in_notes_in_range), these extents are  correct.
 I think it is an issue with pointers and garbage collection of stencils
- does anyone have any intuition about this.

Here's a test file:

\version "2.15.15"

\header {
  texidoc = "LilyPond does in-notes.  The positioning of these notes can
be above or below the staff via the paper variable @code{in note
direction}
and spaced via the variable @{in-note-padding}.
"
}

\relative c' {
  \repeat unfold 5 {
    \repeat unfold 20 { a\< b c d\! }
    % activates the in-note
    \once \override FootnoteItem #'footnote = ##f
    \footnoteGrob #'NoteHead #'(0 . 0)
                  "" \markup { \box \fill-line { "this is a test this is
a test" } }
    a b c d
    \repeat unfold 20 { a b c d }
    \autoFootnoteGrob #'NoteHead #'(-1 . 1) "foo bar foo bar"
    a b c d
   \repeat unfold 20 { a\< b c d\! }
  }
}

Description:
Sketch for in-notes.

Please review this at http://codereview.appspot.com/5293053/

Affected files:
  M lily/constrained-breaking.cc
  M lily/include/constrained-breaking.hh
  M lily/include/page-breaking.hh
  M lily/include/page-layout-problem.hh
  M lily/include/system.hh
  M lily/page-breaking.cc
  M lily/page-layout-problem.cc
  M lily/page-spacing.cc
  M lily/system.cc
  M scm/define-grob-interfaces.scm
  M scm/define-grob-properties.scm
  M scm/define-grobs.scm
  M scm/paper-system.scm