network-manager: Add networkmanagerapplet

This includes nm-connection-editor, which is needed to set up WPA2
enterprise connections.

modules/network-manager.nix

@@ -1,5 +1,9 @@
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 
 lib.mkIf config.sbruder.gui.enable {
   networking.networkmanager.enable = true;
+
+  environment.systemPackages = with pkgs; [
+    networkmanagerapplet
+  ];
 }