Let \time in mid-measure work without warning in some cases (issue 143450043 by address@hidden)

[nine . fierce . ballads]

Reviewers: ,

Message:
This is a simpler (easier to comprehend) change than what I proposed in
http://article.gmane.org/gmane.comp.gnu.lilypond.bugs/39790 .

I rewrote the regression test "time-signature-midmeasure.ly" to focus on
a specific aspect of this feature: suppress the warning, so the output
is quite different.

Description:
Let \time in mid-measure work without warning in some cases

Do not warn as long as the time remaining in the current measure is
less than the length of a measure in the new time signature.  Instead,
preserve the position relative to the beginning of the next measure.
In effect, the current measure is completed as if the old time
signature were still in force.

In other cases, Lilypond still produces a warning.

Please review this at https://codereview.appspot.com/143450043/

Affected files (+192, -25 lines):
  M input/regression/time-signature-midmeasure.ly
  A input/regression/time-signature-midmeasure-3-4-to-4-4.ly
  A input/regression/time-signature-midmeasure-4-4-to-12-8.ly
  A input/regression/time-signature-midmeasure-4-4-to-12-8-triplet.ly
  A input/regression/time-signature-midmeasure-boundary.ly
  A input/regression/time-signature-midmeasure-warning.ly
  M scm/define-music-callbacks.scm