Issue 4947: Link notes to dynamics in Dynamic_performer rather than Staff_performer.

Link notes to dynamics in Dynamic_performer rather than Staff_performer.  This makes dynamics in different voices independent of each other.

This was mentioned in https://codereview.appspot.com/302930043/ .

I doubt that the two new regression tests are very valuable.  If it hadn't been for the previous implementation, I probably would not have thought to create them.  Please vote on whether I should include or omit them when it comes time to push.

[ht, 2016-08-04 20:53:47]

It indeed seems much more logical to associate the Audio_span_dynamic of each
note in Dynamic_performer rather than Staff_performer, and avoid the side
effects caused by identifying Voices by their name in Staff_performer (in the
corner case of identically named Voices).

(I don't really have a strong opinion about whether these side effects could
have originally been intentional or not [as sharing dynamics between Voices
could even have its uses] since, taken in isolation, the previous
Staff_performer code doesn't give any obvious clues about this either way.  As
there are other developers who think the previous behavior is a bug, however, I
have nothing against fixing the behavior like this, and will simplify the
documentation patch issue 4920 accordingly.)

Having regression tests for this functionality could be useful to document the
intended behavior after this change (and might be helpful to anyone trying to
understand the design behind the implementation later).