modularize desktop components

modules/desktop/dm/gdm.nix

@@ -0,0 +1,12 @@
+{ lib, config, ... }:
+let
+  cfg = config.desktop.dm;
+in
+{
+  config = lib.mkIf (cfg.name == "gdm") {
+    services.xserver.displayManager.gdm = {
+      enable = true;
+      wayland = cfg.wayland;
+    };
+  };
+}