at-spi2-core: add dbus module, enabled on gnome3 by default

nixos/modules/module-list.nix

@@ -97,6 +97,7 @@
   ./services/databases/virtuoso.nix
   ./services/databases/monetdb.nix
   ./services/desktops/accountservice.nix
+  ./services/desktops/gnome3/at-spi2-core.nix
   ./services/desktops/gnome3/evolution-data-server.nix
   ./services/desktops/gnome3/sushi.nix
   ./services/desktops/telepathy.nix

nixos/modules/services/desktops/gnome3/at-spi2-core.nix

@@ -0,0 +1,39 @@
+# at-spi2-core daemon.
+
+{ config, pkgs, ... }:
+
+with pkgs.lib;
+
+{
+
+  ###### interface
+
+  options = {
+
+    services.gnome3.at-spi2-core = {
+
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether to enable at-spi2-core, a service for the Assistive Technologies
+          available on the GNOME platform.
+        '';
+      };
+
+    };
+
+  };
+
+
+  ###### implementation
+
+  config = mkIf config.services.gnome3.at-spi2-core.enable {
+
+    environment.systemPackages = [ pkgs.gnome3.at_spi2_core ];
+
+    services.dbus.packages = [ pkgs.gnome3.at_spi2_core ];
+
+  };
+
+}

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

@@ -23,6 +23,7 @@ in {
     security.polkit.enable = true;
     services.udisks2.enable = true;
     services.accounts-daemon.enable = true;
+    services.gnome3.at-spi2-core.enable = true;
     services.gnome3.evolution-data-server.enable = true;
     services.gnome3.sushi.enable = true;
     services.telepathy.enable = true;