Update all usage of lib.concatStrings (lib.intersperse ...) to
lib.concatStringsSep. This produces the same result as per https://github.com/NixOS/nixpkgs/pull/135843,
however it yields a performance benefit on Nix versions that
support the builtins.concatStringsSep primop.
- Added missing freedesktop items (`*.desktop` and icon).
Note that for the moment, only added an `*.svg`
icon as was the only available. It does not give
us the best results (at least in xfce). We may decide
later on to extract the content of the `*.ico` instead
which is a bit more involved.