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>