Support variation in repeated sections

Support variation in repeated sections.

When repeats are unfolded, keep track of the current innermost volta
number in the Timing context.

Add a \conditional music function and predicate generators
(only-in-nth-volta n) and (except-in-nth-volta n).  When the predicate
is false, the conditional music becomes a skip of the same duration.
Example:

  \repeat volta 3 {
    \conditional #(except-in-nth-volta 1) {c^"1st time tacet" ...}
  }

Prior discussion:
http://lists.gnu.org/archive/html/lilypond-devel/2015-01/msg00095.html

All comments are welcome, but be warned that questions along the lines of "Why did you do X?" might not receive a satisfactory answer.  As always, thanks for your time.