Update README.md
Spotimcgui: Updated @run_in_thread kwargs.
Spotimcgui: Playlists: Use taskutils to control background tasks.
Playlists: Loaders: Clear references to tasks when finished.

This avoids creating garbage cycles all over the place.
Views: Artists: Use taskutils to control threads when loading albums.
Spotimcgui: Playback: Use taskutils for playlist generation loops too.
Spotimcgui: Wait for every task before exitting.
Spotimcgui: Enable range requests back.
Spotimcgui: User load_track() from the spotify package.
Spotimcgui: Views: Playlists: Some concurrency related fixes.
Show fallback icons if no album art is available.

This closes #52.
Main Window: Use Player.Art() instead if MusicPlayer.Cover.
Merge branch 'master' into background-support
Spotimcgui: Main Window: Don't minimize on exit.
Main Window: Minimize addon when navigating back on a topmost view.

This closes #121.
Main Window: Flush caches when minimizing.