This has mainly performance benefits, so that we only need to call into
all processors once for every audio buffer segment. It requires
adjusting quite some logic in most processors and in Track, as we have
to consider a larger collection of notes and samples at each step.
There's some cautionary TODOs in the currently unused LibDSP tracks
because they don't do things properly yet.
This was a leftover from the early days of Piano, and there's no reason
to leave it that way especially if we want to use more complex
collection APIs in the future.
* Don't inherit from Core::Object everywhere, that's overkill. Use
RefCounted instead.
* Change some constructor visibilites to facilitate the above.
* default-implement all virtual destructors if possible.
* Drive-by include hygiene.
