hm/graphical-client: enable nm-applet

I'm assuming (probably wrongly) that anything using my graphical-client preset
is _also_ using NetworkManager, which is probably true for real client machines
but may not be true on terminal services machines which also end up with this
preset.

Whatever, I'll work it out later.

ops/nixos/lib/home-manager/graphical-client.nix

@@ -60,6 +60,7 @@
       };
       startup = [
         { command = "xss-lock --transfer-sleep-lock -- i3lock --nofork"; notification = false; }
+        { command = "nm-applet"; notification = false; }
       ];
     };
   };