Motivation for this change

Slim is abandoned and won't work with wayland.
It's in our best interest to use the display-manager
that makes most sense for Plasma5, sddm.

We've already moved on from it being default in #30890
and the graphical.nix profile, which the virtualbox profile uses,
has sddm anyway.

It would also be a good idea, for 20.03, to decide if we want to remove slim entirely.

Additional context

What we have going on with "profiles" here is rather messy.
See #47256, currently the virtualbox appliance has a demo user with a default password, while we've added the nixos user.
I would resolve this but it seems like a hassle, and with #49821 they're now actually documented, which I believe wasn't the best decision because these profiles are working as nixos internals. So now they're like a public API, while being sub-optimal.
So for the foreseeable future I won't contribute a change consolidating this. I'd prefer to keep the installation configuration separate, and something the user cannot leverage publicly.
It already contains dangerous configuration

• https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/installer/cd-dvd/installation-cd-graphical-base.nix#L17

and manually documenting is tedious. Unlike options, there's no generated documentation from values and such. Having something like an mkProfile which has some of the benefits of options would be helpful. That and not having scattered imports.

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 nix-review --run "nix-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.

Notify maintainers

cc @