Issue 4205: Improve part combiner's rest analysis (issue 174610043 by address@hidden)

[nine . fierce . ballads]

Reviewers: ,

Description:
Issue 4205: Improve part combiner's rest analysis

Rests must begin and end simultaneously to be merged into the shared
voice.

Rests, skips, and multi-measure rests are kept apart even if they
begin and end simultaneously.

This does not produce ideal output in every case, but it avoids
producing musical nonsense.

Please review this at https://codereview.appspot.com/174610043/

Affected files (+131, -4 lines):
  A input/regression/part-combine-mmrest-shared.ly
  A input/regression/part-combine-silence.ly
  A input/regression/part-combine-silence-mixed.ly
  M scm/part-combiner.scm