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/app/gaming/gamemode.nix

@@ -1,6 +1,5 @@
 { config, lib, pkgs, ... }:
 
 {
-  environment.systemPackages = [ pkgs.gamemode ];
   programs.gamemode.enable = true;
 }

system/app/gaming/steam.nix

@@ -1,7 +1,6 @@
 { config, lib, pkgs, ... }:
 
 {
-  environment.systemPackages = [ pkgs.steam ];
   programs.steam = {
     enable = true;
     remotePlay.openFirewall = true;