Issue 4600: Let notes/rests suppress multi-measure rest grobs

NOTE: Patch set 1 covers changes already in review.  Please start this review with patch set 2.

As the part combiner shuffles musical content between Voice contexts, it can place notes and rests in a Voice with an ongoing multi-measure rest. The part combiner currently addresses this by killing multi-measure rests at certain times; a major shortcoming of which is that once a rest is killed, it can never reappear.

This enhancement to the Multi_measure_rest_engraver will allow future changes to the part combiner which kill multi-measure rests less often (hopefully never).  (I have already started other improvements; this is not purely conjecture.)

I am submitting this change for testing and review on its own so that there is a clear record of its impact, which should be "no visible difference."

[Dan Eble, 2015-09-12 15:49:20]

Let notes/rests suppress mmrest grobs

[Dan Eble, 2015-09-12 15:57:28]

TIA