Add a Spotify wrapper with a `deviceScaleFactor` argument to set the
`--force-device-scale-factor` flag for high-DPI displays. If unset,
nothing is added.
This allows e.g.
spotify.override { deviceScaleFactor = 1.66; }
Uses a separate wrapper derivation as suggested by @Ma27.
Uses `wrapProgram` instead of `makeWrapper` as suggested by @Ma27.
