Motivation for this change

This is PRed for visibility. I have seen a couple of times people ask about lightdm-web-greeter. This is not ready to merge, though it works. This will give a hand to anyone trying to use or contribute lightdm-web-greeter, as they are bound to find this PR.

Why 3.0.0rc2 and not stable 2.2.5?

I did an initial try in the past, for the 2.x branch. It will not work with nixos. It seems that there is a bug with the way webkitgtk is packaged for NixOS, and local files are not rendered as html, but as plain text. This seems to also affect gnucash and the same bug did affect midori's DOM inspector during at least 17.09's lifetime.

Why isn't this PR for 3.0.0rc2 finished?

Well, first thing first, the code quality for the module hasn't been reviewed; this is a port from a module I wrote about a year ago, which was basically a copy of the ligthdm-gtk-greeter module. I would also need to take a couple minutes to review the derivations, but I think they're in mostly good standing.

Then, there's the issue of "rc2" It has been tagged 6 months ago, on the 8th of January. I would much prefer if they tagged a stable release. Though, there is some activity in the repo, so they eventually will.

Finally, the main reason I'm not continuing: I'm not sure about the quality of the implementation of that greeter. Maybe it's lightdm's configuration at fault, and not the web greeter, but it seems a bit buggy. The shutdown/reboot buttons do nothing (inside a nixos-rebuild build-vm). There are also some layout issue in the default theme, but that's probably not as much an issue.

I'm more concerned with the current state of their whither library, which I had to patch to work around (their quote) "broke[n] file::// url access in qt 5.11". I had to basically remove a bit of security from the library for it to work. That's not good. Until that is resolved, I may continue on and off (mostly off) to polish this, but this isn't worthy yet for inclusion.

Things done

• ✔️ Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-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)
• ⬜ Fits CONTRIBUTING.md.