Huge structural changes
Well, this commit got a bit out of hand, what with 26 files changed. Oh well.

Basically, we're using the buffermanager to dispense temporary buffers for playhandles and audioports to use.
This allows us to change the way playhandles work. Earlier, playhandles of the same track were waiting in line
to push their output to the audioport. This was of course inefficient, so now they just register themselves to the port,
then the port handles mixing the buffers.

Caveat: this is still a work in progress, the vol/pan knobs on instruments are temporarily non-functional - will be fixed in
the next commit, but I have to get some sleep now.