Allow to disable Jack auto connect

I am using a jack session manager (Ray Session) to manage jack connections.
With LMMS, I always have direct jack connections to output made by LMMS.

Add a button un jack preferences to allow to disable jack autoconnection.

Use LMMS's JACK preferences, the only entry point named in the issue, as the starting point. Done means the preferences include a control for disabling JACK autoconnection and that the setting prevents LMMS from making its direct output connections when a session manager such as Ray Session is in use.

Written by the indexing model from the issue text.