free42: add desktop item

[plabadens]

Motivation for this change

Add a desktop item for the free42dec variant of Free42. It's unclear to me whether another one should be made for free42bin. Maybe it would be preferable to add a package option to select between the two?

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• 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 nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[plabadens]

I'm unsure what's the best way of exposing free42dec and free42bin in desktop items. Would it make sense to add two package options and allow users to select which binaries to build? In that case we'd have one desktop item for each variant. On the other hand, I would assume most users don't need free42bin, as precision is more important than performance in a calculator app.

@AndersonTorres what do you think is preferable?

[SuperSandro2000]

Two desktop icons for one package is fine. We don't need to clutter the package list with such small decisions.

[AndersonTorres]

Two desktop items, one for each binary file.

After all, the other distros build the two executables. The end-user is the one to worry about those minor practical issues.

[plabadens]

Sounds good, I'll keep it simple and just add a second desktop item for free42bin.

[plabadens]

Alright, both binaries have desktop icons now.