Add KDE and SDDM display manager.

nodes/hosts/common/services/desktopmanager.nix

@@ -0,0 +1,3 @@
+{ ... }: {
+  services.desktopManager.plasma6.enable = true;
+}

nodes/hosts/common/services/displaymanager.nix

@@ -0,0 +1,3 @@
+{ ... }: {
+  services.displayManager.sddm.enable = true;
+}