nixos: Modularize NixOS and Home Manager config

modules/desktop/firefox.nix

@@ -0,0 +1,3 @@
+{
+  programs.firefox.enable = true;
+}

modules/desktop/plasma.nix

@@ -0,0 +1,10 @@
+{ ... }: 
+{
+  services.desktopManager.plasma6.enable = true;
+  services.displayManager = {
+    plasma-login-manager.enable = true;
+    autoLogin = { enable = true; user = "emmatherock"; };
+  };
+  security.pam.services.login.enableKwallet = true;
+
+}