Use XDG_STATE_HOME for jackdbus logs

[keithbowes]

Fixes #402

In that bug, it's mentioned that Debian has an $XDG_STATE_HOME extension for that purpose. Well, it's been added to version 0.8 of the specification (8 May 2021) with the description:

The $XDG_STATE_HOME contains state data that should persist between (application) restarts, but that is not important or portable enough to the user that it should be stored in $XDG_DATA_HOME. It may contain:

• actions history (logs, history, recently used files, …)
• current state of the application that can be reused on a restart (view, layout, open files, undo history, …)

It's been a part of the official specification for almost three years and other applications use it, so it should be safe to use it and not add unnecessary files and directories to $HOME.