nixos/gnome: Add the NixOS background metadata

This is needed to have it listed in the Control Center.

nixos/modules/services/x11/desktop-managers/gnome.nix

@@ -22,6 +22,9 @@ let
     favorite-apps=[ 'org.gnome.Epiphany.desktop', 'org.gnome.Geary.desktop', 'org.gnome.Calendar.desktop', 'org.gnome.Music.desktop', 'org.gnome.Photos.desktop', 'org.gnome.Nautilus.desktop' ]
   '';
 
+  nixos-background-ligtht = pkgs.nixos-artwork.wallpapers.simple-blue;
+  nixos-background-dark = pkgs.nixos-artwork.wallpapers.simple-dark-gray;
+
   nixos-gsettings-desktop-schemas = let
     defaultPackages = with pkgs; [ gsettings-desktop-schemas gnome.gnome-shell ];
   in
@@ -42,11 +45,11 @@ let
      chmod -R a+w $out/share/gsettings-schemas/nixos-gsettings-overrides
      cat - > $out/share/gsettings-schemas/nixos-gsettings-overrides/glib-2.0/schemas/nixos-defaults.gschema.override <<- EOF
        [org.gnome.desktop.background]
-       picture-uri='file://${pkgs.nixos-artwork.wallpapers.simple-blue.gnomeFilePath}'
-       picture-uri-dark='file://${pkgs.nixos-artwork.wallpapers.simple-dark-gray.gnomeFilePath}'
+       picture-uri='file://${nixos-background-ligtht.gnomeFilePath}'
+       picture-uri-dark='file://${nixos-background-dark.gnomeFilePath}'
 
        [org.gnome.desktop.screensaver]
-       picture-uri='file://${pkgs.nixos-artwork.wallpapers.simple-dark-gray-bottom.gnomeFilePath}'
+       picture-uri='file://${nixos-background-dark.gnomeFilePath}'
 
        ${cfg.favoriteAppsOverride}
 
@@ -56,6 +59,26 @@ let
      ${pkgs.glib.dev}/bin/glib-compile-schemas $out/share/gsettings-schemas/nixos-gsettings-overrides/glib-2.0/schemas/
     '';
 
+  nixos-background-info = pkgs.writeTextFile rec {
+    name = "nixos-background-info";
+    text = ''
+      <?xml version="1.0"?>
+      <!DOCTYPE wallpapers SYSTEM "gnome-wp-list.dtd">
+      <wallpapers>
+        <wallpaper deleted="false">
+          <name>Blobs</name>
+          <filename>${nixos-background-ligtht.gnomeFilePath}</filename>
+          <filename-dark>${nixos-background-dark.gnomeFilePath}</filename-dark>
+          <options>zoom</options>
+          <shade_type>solid</shade_type>
+          <pcolor>#3a4ba0</pcolor>
+          <scolor>#2f302f</scolor>
+        </wallpaper>
+      </wallpapers>
+    '';
+    destination = "/share/gnome-background-properties/nixos.xml";
+  };
+
   flashbackEnabled = cfg.flashback.enableMetacity || length cfg.flashback.customSessions > 0;
   flashbackWms = optional cfg.flashback.enableMetacity {
     wmName = "metacity";
@@ -431,6 +454,7 @@ in
       # Adapt from https://gitlab.gnome.org/GNOME/gnome-build-meta/blob/gnome-3-38/elements/core/meta-gnome-core-shell.bst
       environment.systemPackages = with pkgs.gnome; [
         adwaita-icon-theme
+        nixos-background-info
         gnome-backgrounds
         gnome-bluetooth
         gnome-color-manager