node-editor: Make this app non-unique

There's no benefit in having multiple windows share the process.

But there's a huge disadvantage because running the app a 2nd time with
different environment variables will open a window in the first process
instead and discard the variables.

And my use of GSK_RENDERER hates that.

demos/node-editor/node-editor-application.c

@@ -299,7 +299,7 @@ node_editor_application_new (void)
 
   app = g_object_new (NODE_EDITOR_APPLICATION_TYPE,
                       "application-id", "org.gtk.gtk4.NodeEditor",
-                      "flags", G_APPLICATION_HANDLES_OPEN,
+                      "flags", G_APPLICATION_HANDLES_OPEN | G_APPLICATION_NON_UNIQUE,
                       NULL);
 
   g_application_add_main_option (G_APPLICATION (app), "version", 0, 0,G_OPTION_ARG_NONE, "Show program version", NULL);