Fix memory leaks

* Fix recursive reference memory leak in UIWidget
* Make Event/ScheduledEvent memory-leak safe
* Fix exit crashs by freeing graphics resources before destroying GL context
* Add many asserts to avoid any leak regression

src/main.cpp

@@ -28,6 +28,7 @@ int main(int argc, const char* argv[])
     OTClient app;
     app.init(args);
     app.run();
+    app.deinit();
     app.terminate();
     return 0;
 }