[desktop] Refactor into multiple components

roles/desktop/kde.nix

@@ -0,0 +1,12 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.roles.desktop;
+in
+{
+  config = mkIf (cfg.enable && cfg.kde) {
+    services.desktopManager.plasma6.enable = true;
+  };
+}