Issue 2445: Add measure counter to LilyPond (issue 6730044)

[david . nalesnik]

Reviewers: ,

Message:
Please review.

Thanks!

-David

Description:
Issue 2445: Add measure counter to LilyPond

This patch allows the user to number groups of successive measures,
which
is useful as an aid to the performer in keeping track of repetitions.
There
is no requirement that the counted measures be repeated, however.  The
number is centered on the measure (discounting prefatory material),
above
the staff by default.

A new grob, a spanner called `MeasureCounter,' is created.  Each element
of
a count is an independent instance of this grob.  A new event-class,
`measure-
counter-event,' is also defined.

At present, the engraver, `Measure_counter_engraver,' must be added
to the relevant context (probably Staff) in the .ly file.  This engraver
has been written in Scheme, following the precedent of
`Span_stem_engraver.'

The user uses the commands `\startMeasureSpanner' and
`\stopMeasureSpanner'
to frame the measures to count.  The initial number is read from a
new grob-property, 'count-from, which may be overridden.

Please review this at https://codereview.appspot.com/6730044/

Affected files:
  A input/regression/measure-counter.ly
  M ly/spanners-init.ly
  M scm/define-event-classes.scm
  M scm/define-grob-interfaces.scm
  M scm/define-grob-properties.scm
  M scm/define-grobs.scm
  M scm/define-music-types.scm
  M scm/music-functions.scm