Adding back AppKit

[michaelmior]

I noticed in this fork that the AppKit backend using NSSound was removed. Any particular reason? This seems to work better for me in the original playsound.

[sjmikler]

I had limited ways of testing Mac version (only using a VM) and it seemed to me afplay would be more robust. If you say so, I will retest NSSound and add it as an optional (maybe default?) backend.

Can you help me understand in what way it works better than afplay?

[michaelmior]

@sjmikler It avoids having to fork another process which is nice. It also opens the possibility of controlling the sound, even though this is not something the library currently does.