playlist: Fix another copy-paste error in docs
playlist: Add tracks_added callback
playlist: Add tracks_removed callback
playlist: Add tracks_moved callback
playlist: Add playlist_state_changed callback
playlist: Add playlist_update_in_progress callback
playlist: Add playlist_metadata_updated callback
playlist: Add track_created_changed callback
playlist: Add track_seen_changed callback
playlist: Add description_changed callback
playlist: Add image_changed callback
playlist: Add track_message_changed callback
playlist: Add subscribers_changed callback