nixos/gnome: install xdg-user-dirs{,-gtk} systemd units

GNOME 49 has completely removed the built-in service manager, which allowed
.desktop files to be used for launching services. This mechanism was used by
xdg-user-dirs and xdg-user-dirs-gtk to update the xdg user directories.

In recent versions of xdg-user-dirs and xdg-user-dirs-gtk, systemd units are
included in the installation to work around that. This commit installs those
units when gnome-shell is enabled.

Link: https://gitlab.gnome.org/GNOME/gnome-session/-/blob/49.2/NEWS?ref_type=tags#L51-54
Link: https://gitlab.freedesktop.org/xdg/xdg-user-dirs/-/commit/2a63d3f0ffb76135790bb1168a3270a599904380

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

@@ -390,6 +390,10 @@ in
       systemd.packages = [
         pkgs.gnome-session
         pkgs.gnome-shell
+      ]
+      ++ removeExcluded [
+        pkgs.xdg-user-dirs # Update user dirs as described in https://freedesktop.org/wiki/Software/xdg-user-dirs/
+        pkgs.xdg-user-dirs-gtk # Used to create the default bookmarks
       ];
 
       services.udev.packages = [