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Maybe that could offer the basis for an alternative client. Thanks for sharing your knowledge. :)

An alternative desktop client using libspotify and a non-web GUI toolkit would be much more interesting.

Very true. There are a few options in that space: Clementine (https://www.clementine-player.org/), Tomahawk (https://www.tomahawk-player.org/), Mopidy (https://www.mopidy.com/), Fidelify (http://fidelify.net/, not OSS), Soundbounce (http://soundbounce.org/). (Raise a glass to SOVND, my personal project that I killed when I got fed up with libspotify.)

Tried the first two: they are pretty poor music player as it is in my opinion. The rest seems to be much more that a Spotify client.

A plugin for WinAmp or Foobar would be great.