gnome-disks: add D-Bus service

nixos/modules/module-list.nix

···
   ./services/desktops/geoclue2.nix
   ./services/desktops/gnome3/at-spi2-core.nix
   ./services/desktops/gnome3/evolution-data-server.nix
+  ./services/desktops/gnome3/gnome-disks.nix
   ./services/desktops/gnome3/gnome-documents.nix
   ./services/desktops/gnome3/gnome-keyring.nix
   ./services/desktops/gnome3/gnome-online-accounts.nix

nixos/modules/services/desktops/gnome3/gnome-disks.nix

···
+# GNOME Disks daemon.
+
+{ config, pkgs, lib, ... }:
+
+with lib;
+
+let
+  gnome3 = config.environment.gnome3.packageSet;
+in
+{
+
+  ###### interface
+
+  options = {
+
+    services.gnome3.gnome-disks = {
+
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether to enable GNOME Disks daemon, a service designed to
+          be a UDisks2 graphical front-end.
+        '';
+      };
+
+    };
+
+  };
+
+
+  ###### implementation
+
+  config = mkIf config.services.gnome3.gnome-disks.enable {
+
+    environment.systemPackages = [ gnome3.gnome-disk-utility ];
+
+    services.dbus.packages = [ gnome3.gnome-disk-utility ];
+
+  };
+
+}

nixos/modules/services/x11/desktop-managers/gnome3.nix

···
     services.geoclue2.enable = mkDefault true;
     services.gnome3.at-spi2-core.enable = true;
     services.gnome3.evolution-data-server.enable = true;
+    services.gnome3.gnome-disks.enable = mkDefault true;
     services.gnome3.gnome-documents.enable = mkDefault true;
     services.gnome3.gnome-keyring.enable = true;
     services.gnome3.gnome-online-accounts.enable = mkDefault true;