winrt: Touch events stop working after initiating drag via touch.

When starting a drag via touch, after the drag has finished, touch events stop working in the app.

I think it because Windows does not generate a PointerExited event when an UWP dragging session is started. This then leaves the Qt touch tracking in an inconsitant state, and subsequent touch begins do not get handled correctly.

I have attached an demo app that reproduces the issue. Please do the following:

1. Run the attached app.
2. Confirm the "Test 1" and "Test 2" buttons respond to touch.
3. Using touch, press and drag the red square to start the UWP drag session.
4. End the drag.
5. Touch the "Test 1" and "Test 2" buttons and they should now not respond to touch.