artist: Add ArtistBrowserType enum
artist: Add ArtistBrowser with most methods
artist: Add ArtistBrowser.tophit_tracks
artist: Add ArtistBrowser.albums
artist: Add ArtistBrowser.similar_artists
artist: Add ArtistBrowser.portraits
album, artist: Remove no longer relevant TODOs
album, artist: Fix behavior when not loaded
utils: Better docstring wording
album, artist: Test setting of complete_event
inbox: Add InboxPostResult.__repr__()
Add add_ref kwarg to all objects for consistency
docs: Extract release version from code
playlist: offline_status works when not loaded
playlist: Delete both ends of the folder, optionally with contents
playlist: Test _find_folder_indexes helper
playlist: Simplify docstring
docs: Extend development plans
track: Add review TODO
lib: Serialize all libspotify access to increase thread-safety