listenbrainz: submit duration

Extend the listenbrainz plugin to also submit the duration. This might be used by ListenBrainz to improve matching the submitted track to MusicBrainz and also for deciding on how to keep the "playing now" notification active.

See also:

• https://listenbrainz.readthedocs.io/en/latest/users/json.html#payload-json-details
• https://community.metabrainz.org/t/possible-bug-found-listening-now-notification-disappears-after-listening-to-songs-longer-than-10-minutes/598239