Add GNOME

- Add config option to switch between window managers / desktop environments
- Remove systemPackages attribute when a service or program attribute is available
- Add GNOME to gui

system/hardware/printing.nix

@@ -1,11 +1,6 @@
 { config, lib, pkgs, ... }:
 
 {
-  environment.systemPackages = [
-    pkgs.cups
-    pkgs.cups-filters
-    pkgs.avahi
-  ];
   services.printing.enable = true;
   services.avahi.enable = true;
   services.avahi.nssmdns4 = true;