Added flatpak

I would like to get this a little simpler so you don't have to manually
add the remote. But this is pretty trivial to do. I'll revisit this later.

modules/system/default.nix

@@ -1 +1 @@
-{ config, pkgs, ... }: { imports = [ ./gtk.nix ./xdg.nix ]; }
+{ config, pkgs, ... }: { imports = [ ./gtk.nix ./xdg.nix ./flatpak.nix ]; }

modules/system/flatpak.nix

@@ -0,0 +1,25 @@
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.systems.flatpak;
+  graphical = config.graphical;
+in with lib; {
+  options = {
+    systems.flatpak = {
+      enable = mkOption {
+        default = graphical.enable;
+        type = with types; bool;
+        description = "When they tried to do docker for GUIs";
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    services.flatpak.enable = true;
+    xdg.portal = {
+      enable = true;
+      extraPortals =
+        [ pkgs.xdg-desktop-portal-gtk pkgs.xdg-desktop-portal-wlr ];
+    };
+  };
+
+}