nixos/gnome3: additions to core-shell

Adds:

- gnome-color-manager
- services.avahi
  It appears that GeoClue requires its daemon and IIRC has
  been default enabled in other distros for a while.
- orca
  It's the default screen-reader.

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

@@ -238,6 +238,8 @@ in
       services.dbus.packages =
         optional config.services.printing.enable pkgs.system-config-printer;
 
+      services.avahi.enable = mkDefault true;
+
       services.geoclue2.enable = mkDefault true;
       services.geoclue2.enableDemoAgent = false; # GNOME has its own geoclue agent
 
@@ -261,16 +263,19 @@ in
         source-sans-pro
       ];
 
+      # Adapt from https://gitlab.gnome.org/GNOME/gnome-build-meta/blob/gnome-3-32/elements/core/meta-gnome-core-shell.bst
       environment.systemPackages = with pkgs.gnome3; [
         adwaita-icon-theme
         gnome-backgrounds
         gnome-bluetooth
+        gnome-color-manager
         gnome-control-center
         gnome-getting-started-docs
         gnome-shell
         gnome-shell-extensions
         gnome-themes-extra
         gnome-user-docs
+        pkgs.orca
         pkgs.glib # for gsettings
         pkgs.gnome-menus
         pkgs.gtk3.out # for gtk-launch