Listenbrainz support a compatibility for last.fm scrobbler, so a bare minimum adding this is very easy - just change the post URL. It copy-pastes ~300 lines of code. Dirty, but works. Better alternatives are a topic of discussion:
Add URL field in scrobbler settings (and rename old last.fm plugin to some more generic name like "Audio scrobble"?), this will not allow having multiple scrobblers at once.
Rename last.fm plugin to generic "Audio scrobble" and introduce a multiple url+username+password sets for handling sumbits. Might save the code duplication, but complicate settings a slight bit.
Keep plugin separate, and implement listenbrainz API, don't use listenbrainz proxy compatibility. This would still have code duplication on firing/listening events.
Listenbrainz support a compatibility for last.fm scrobbler, so a bare minimum adding this is very easy - just change the post URL. It copy-pastes ~300 lines of code. Dirty, but works. Better alternatives are a topic of discussion:
Note: I'm basing this from #257, my original progress and more comments are tracked on forked repo - https://github.com/carl-di-ortus/gmusicbrowser/issues/2
I intend to keep this draft pull request hanging here, just to notify my intention, and gather feedback.