Move jack_control command handlers to own functions

[dj-foxxy]

I've tried to change as little as possible within the handlers. I've switched our setup to use this version so will be testing it.

[nedko]

This commit breaks python2 compatibility of jack_control

While I like the intent, in LADI/jackdbus I'm playing conservative and keeping python2 compatibility, for its stable interface (even if it has other issues) and thus the non-deployability to python2 systems is a concern.

[falkTX]

main jack2 doesnt care about python2, any recent enough distribution does not provide it as it is unmaintained by a very long time and is now a security risk.

there was some initial work to move from waf to meson, which will help with this.

[dj-foxxy]

Hi all. We've moved away from JACK so I'm unlikely to work on this PR. Feel free to close. :)