nixos/xdg: add portal option

This factors the configuration out of the flatpak module.

nixos/modules/config/xdg/portal.nix

@@ -0,0 +1,38 @@
+{ config, pkgs ,lib ,... }:
+with lib;
+{
+  options.xdg.portal = {
+    enable =
+      mkEnableOption "<link xlink:href='https://github.com/flatpak/xdg-desktop-portal'>xdg desktop integration</link>"//{
+        default = true;
+      };
+
+    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 and KDE already
+        adds <package>xdg-desktop-portal-gtk</package>; and
+        <package>xdg-desktop-portal-kde</package> respectively. On other desktop
+        environments you probably want to add them yourself.
+      '';
+    };
+  };
+
+  config =
+    let
+      cfg = config.xdg.portal;
+      packages = [ pkgs.xdg-desktop-portal ] ++ cfg.extraPortals;
+
+    in mkIf cfg.enable {
+
+      services.dbus.packages  = packages;
+      systemd.packages = packages;
+      environment.variables = {
+        GTK_USE_PORTAL = "1";
+        XDG_DESKTOP_PORTAL_PATH = map (p: "${p}/share/xdg-desktop-portal/portals") cfg.extraPortals;
+      };
+    };
+}