Add Duration Option to Custom Filters

A minor impact: The Plex app on macOS and also the web interface now show Something went wrong -- An unexpected error occurred. when trying to edit a filter that has had Track Duration added via PlexAmp.

I can confirm this - “Track duration” is not longer a working filter, and all playlists with it as an option crash when I try to edit them.

Tried this on web. Still works in the Plexamp app though!