Allow music-wrapper-music as toplevel of \parallelMusic

Allow music-wrapper-music as toplevel of \parallelMusic

Addresses issue 451 by allowing something like
\parallelMusic #'(a b c)
\relative c' { ... }

Whether this is actually useful for anything is a different question:
I think that the proposals in the issue itself (and what made it into
the NR), namely applying \relative to each of the music variables
after splitting up, are likely more convenient than having each bar
start relative to the preceding one from a different voice.

Slightly less pointless would likely be something like
\parallelMusic #'(a b c)
\transpose ...
which should now work as well.

Anyway, I just tackled this one in an idle moment because I wrongly
remembered it as having a bounty attached.

And might as well get another issue closed if nothing else.