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/gui/lightdm.nix

@@ -1,8 +1,5 @@
 { config, lib, pkgs, ... }:
 
 {
-  environment.systemPackages = [ pkgs.lightdm ];
-  services.xserver.displayManager = {
-    lightdm.enable = true;
-  };
+  services.xserver.displayManager.lightdm.enable = true;
 }