Route native close through platform services

src/app.h

@@ -189,6 +189,7 @@ public:
     void pick_dir(std::function<void(std::string path)> callback);
     void display_file(std::string path);
     void share_file(std::string path);
+    void request_app_close();
     void save_prepared_file(
         std::string path,
         std::string suggested_name,