Issue 4233: Improve partcombine multi-measure rest handling.

So, for the first time in about eight years, I compose a piece in which
I want to use the plain old partcombine command, and it happens to
demonstrate some known deficiencies.  This scratches the itch.

Add "silence1" and "silence2" states which constrain the partcombine
iterator to take events from a specific part during intervals of
silence.  These new states allow the following improvements.

Detect when a multi-measure rest starts in the middle of a
multi-measure rest in the other voice and engrave it as a shared rest.
Previously, the new rest would have been engraved as a voice-specific
rest.

In solo analysis, when a rest and a multi-measure rest begin
simultaneously, engrave the rest as a shared rest and omit the
multi-measure rest.  Previously, both rests would have been engraved
as voice-specific rests.

A regression test has been changed just to remove "!!!" markers which
were drawing attention to recognized undesirable output that is no
longer a problem.

[Dan Eble, 2014-12-29 04:19:38]

style

[Dan Eble, 2014-12-29 14:28:45]

revise texidoc in regtest