Windows Pointer API support uses PointerId for QTabletEvent::uniqueId

Qt uses POINTER_INFO::pointerId for https://doc.qt.io/qt-5/qtabletevent.html#uniqueId. The documentation says that uniqueId "returns a unique ID for the current device, making it possible to differentiate between multiple devices being used at the same time on the tablet."

As noted in https://bugs.kde.org/show_bug.cgi?id=405494#c14, POINTER_INFO::sourceDevice might be a better choice, since pointerId changes every time the pen approaches the tablet, which means we cannot use it to recognize the stylus.