sway: Install the wallpapers by default and use them on NixOS

[primeos]

Size impact:
$ du -sh $(nix-build -A sway-unwrapped)
5.9M /nix/store/d07zcpybgpdj8xhn9n4qb04lgm453ia1-sway-unwrapped-1.6
nix@sessel:/var/tmp/michael/nixpkgs$ du -sh $(nix-build -A sway-unwrapped)/share/backgrounds
4.9M /nix/store/d07zcpybgpdj8xhn9n4qb04lgm453ia1-sway-unwrapped-1.6/share/backgrounds

Screenshot:

Motivation for this change

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.

[primeos]

I also considered "-Ddefault-wallpaper=${boolToString isNixOS}" but I think the best approach might be to add a wallpaper output (and add that to the NixOS module - which makes it still difficult to override but 5 MB are hopefully not that bad - or we keep it as it is).

[primeos]

FWIW Arch installs them by default so I guess this should actually be fine: https://archlinux.org/packages/community/x86_64/sway/

[grahamc]

It seems to me we may want to instead default it to using NixOS themed wallpapers?

[primeos]

@grahamc that's a good question. Personally I prefer sticking to the upstream defaults but if we already default to NixOS themed wallpapers for other compositors/DEs/WMs (e.g. Gnome/KDE) then it might be best to do the same for Sway.

[Synthetica9]

I somehow feel it'd be pretty weird if the non-NixOS install came with custom wallpapers... I'd be fine with a Nix wallpaper on NixOS tho

[Synthetica9]

Sure thing!