refactor: Enable unreachable cleanup logic in main()

> refactor `main_implemntation()` cleanup to run once on `launcher::shutdown()`, not sure why it was defined like that (unreachable code, at least for iidx)
> not thread safe, but probably not needed (no obvious failures as well)
> related to #1

launcher/shutdown.cpp

@@ -15,11 +15,15 @@
 
 namespace launcher {
 
+    std::function<void()> on_shutdown = []() {};
+
     void stop_subsystems() {
         // note that it is possible for stop_subsystems to be called multiple times
         // (e.g., crashing, and then closing the window)
         // therefore, subsystems need to be guarded against multiple unload attempts
         log_info("launcher", "stopping subsystems");
+        on_shutdown(); //_BUG: not thread safe, required?
+        on_shutdown = []() {};
 
         // flush/stop logger
         logger::stop();