Support for audtool (command line interface)?

There are two ways that I think are useful to do this:

Adding `--own-name=org.atheme.audacious=own` would allow an existing audtool binary (or other user of the dbus interface) that is running outside the flatpak to control audacious regardless of where it's running. You can use `/var/lib/flatpak/app/org.atheme.audacious/current/active/files/bin/audtool` to test outside the flatpak.

I'm not sure if this is easy to to with flatpak, but providing a way to easily invoke audtool from the outside would also be useful. `flatpak enter org.atheme.audacious audtool` does work, but it's not very discoverable. In particular, a wrapper that also adds the equivalent of `--file-forwarding` would be useful.

Start by testing /var/lib/flatpak/app/org.atheme.audacious/current/active/files/bin/audtool outside the Flatpak, then compare it with `flatpak enter org.atheme.audacious audtool`. The work is done when audtool can be invoked externally and control Audacious, ideally with equivalent file-forwarding behavior; the issue leaves the implementation approach open.

Written by the indexing model from the issue text.