winrt: Exit application properly

After returning from main() it is not sufficient to exit the main
thread, it also needs to be ensured that the Xaml::IApplication object
gets notified by invoking Exit.

Task-number: QTBUG-49141
Change-Id: I8ca434be5f17ddddd465ede2a79585c28c51b3ef
Reviewed-by: Andrew Knight <andrew.knight@intopalo.com>

src/winmain/qtmain_winrt.cpp

@@ -156,7 +156,9 @@ public:
             AppContainer *app = reinterpret_cast<AppContainer *>(param);
             int argc = app->args.count();
             char **argv = app->args.data();
-            return main(argc, argv);
+            const int res = main(argc, argv);
+            app->core->Exit();
+            return res;
         }, this, CREATE_SUSPENDED, nullptr);
 
         HRESULT hr;