HUGE REFACTOR OF FILTER GENERICS

They now allow dynamic modification of input channels. The interface for our
elements has thusly changed, and this required rewriting elements of every
filter. OutputChannel was also renamed to simply Channel during this change.
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).