moved common out of systems

common/optional/desktop.nix

@@ -0,0 +1,12 @@
+{
+  services = {
+    desktopManager.plasma6.enable = true;
+    xserver = {
+      enable = true;
+      xkb = {
+        layout = "us";
+        variant = "";
+      };
+    };
+  };
+}