nixos/xdg: add portal option

This factors the configuration out of the flatpak module.

nixos/modules/services/desktops/flatpak.nix

@@ -15,38 +15,22 @@ in {
   options = {
     services.flatpak = {
       enable = mkEnableOption "flatpak";
-
-      extraPortals = mkOption {
-        type = types.listOf types.package;
-        default = [];
-        description = ''
-          List of additional portals to add to path. Portals allow interaction
-          with system, like choosing files or taking screenshots. At minimum,
-          a desktop portal implementation should be listed. GNOME already
-          adds <package>xdg-desktop-portal-gtk</package>; for KDE, there
-          is <package>xdg-desktop-portal-kde</package>. Other desktop
-          environments will probably want to do the same.
-        '';
-      };
     };
   };
 
 
   ###### implementation
   config = mkIf cfg.enable {
+
     environment.systemPackages = [ pkgs.flatpak ];
 
-    services.dbus.packages = [ pkgs.flatpak pkgs.xdg-desktop-portal ] ++ cfg.extraPortals;
+    services.dbus.packages = [ pkgs.flatpak ];
 
-    systemd.packages = [ pkgs.flatpak pkgs.xdg-desktop-portal ] ++ cfg.extraPortals;
+    systemd.packages = [ pkgs.flatpak ];
 
     environment.profiles = [
       "$HOME/.local/share/flatpak/exports"
       "/var/lib/flatpak/exports"
     ];
-
-    environment.variables = {
-      XDG_DESKTOP_PORTAL_PATH = map (p: "${p}/share/xdg-desktop-portal/portals") cfg.extraPortals;
-    };
   };
 }