gnome3.gpaste: general improvements
gnome3.gpaste: general improvements

* Port to Meson (builds much faster)
   * needed a patch to install systemd units
   * soversion does not match but we do not care (https://github.com/Keruspe/GPaste/pull/285)
* Format with nixpkgs-format
* Updated license (BSD since 3.20)
* Drop dbus dependency (we are setting the paths manually anyway)
* Switch to fetchFromGitHub
* Add comment why clutter is needed

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