colorpicker: mention more alternatives

[gepbird]

After the removal of colorpicker, the listed alternatives were not sufficient for me. It was a lightweight "CLI" app in the sense that it didn't open any windows, just printed the color where you clicked next (and with --preview it drew the color you hovered over to the bottom left corner). The mentioned alternatives open a GUI, require an extra button to click to pick colors, doesn't work on minimal systems due to some freedesktop errors, and presumably don't print the color in the terminal, so they can't be used in scripts.

xcolor follows the simplicity of colorpicker so it is a closer alternative and probably other ex-users will want to migrate to that. I don't see much value to deleting the previously mentioned GUI color pickers like eyedropper and gcolor3, so at that point I also added a more featureful GUI alternative called gpick.

cc @Aleksanaa for removing colorpicker in #349744.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[Aleksanaa]

Thanks.