Starting work for polyphonic instrument generic

There isn't an elegant way to do this without the refactor I want to do with
runtime-modifiable filters, so I will finish this without the other refactor.
Merge branch 'master' into polyphonicinst

Conflicts:
	src/subtractive_synth.cpp
	src/subtractive_synth.h

With the refactor in place I was also able to add the last couple of needed
changes to the interface for polyphonic instruments. This completes the
polyphonic refactor (issue #5).