Motivation for this change

When working on a headless server, it is convenient to disable installation of graphical frontends via a single configuration setting. I've only found noXlibs to do that, so I've enriched it with a bunch of package overrides.
Note that it may not be a good idea to have noXlibs depend on app derivations. Maybe a better way is to invert the dependency: feed a noXlibs flag to app derivations so that they can decide what to disable.

Things done

Please check what applies. Note that these are not hard requirements but mereley serve as information for reviewers.

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.