noctalia-greeter: init

[rwxae]

Add support for noctalia-greeter.

Things to note:

1. This implementation adds theming for standalone version only. But since very recently there is a built-in NixOS module¹ for noctalia-greeter. All we need to do is to update flake.lock and reuse these options in services.displayManager.noctalia-greeter.
2. Logo looks weird with light themes. After digging into the code, I noticed that noctalia-greeter tints the logo with the on_surface color, which is usually very dark on light themes. I think it is the issue on the noctalia-greeter's side. Maybe we should open an issue.

Some screenshots:

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¹. Btw, there is now a module for Noctalia itself (but in home-manager). I think we can drop the lib.optionals (options.programs ? noctalia) check.

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