Add PCM sampling instrument

[denjhang]

😜

[freq-mod]

Actually, it has already been reported and discussed on libOPNMIDI side, but an actual work is yet to be done, sadly.

UI would have to be redone somewhat, too. A sketch of what things would look like (sketch-psg-and-wt branch) was made irrelevant by PR #80. As much as I would love to help in finishing this, but I can't do much, at least with my current skills. I can only leave this bad mockup:

[denjhang]

If you can implement the functions in the diagram, your work will surpass bamboo tracker and deflemack, and lead us to the era of efficient VGM production. How exciting!