Fix file browser in qmlvideofx example.

Similar to de9092389f2e43370c2cfcd6759d08cc11da9a68 for qmlvideo.
Use QUrl::fromLocalFile() to get Windows drive handling
right. Emulate its behavior in QML code. Fix up() to terminate
correctly.

Task-number: QTBUG-32139
Change-Id: I36bafaa608ff054190dc76694f6254a74f3b513e
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>