remove JACK related menu items (audio/MIDI setup is where this happens now)
change return type on {disconnect_from,reconnect_to}_engine() and make public so that we can use these when doing intentional disconnects without popping up the "HALTED" dialog
remove JACK related action definitions (audio/MIDI setup is where this happens now)
substantive changes to allow the audio/MIDI setup dialog to change settings as intended

So far only tested with JACK backend, but expected to be more correct for other backends too
minor code changes