Set StrongFocus on QQuickWidget

Make text input functional out of the box.

Both QGraphicsView and QDeclarativeView (QQuick1) do this so there
is no reason to do this differently for QQuickWidget. For QQuickWindow
and QQuickView the issue does not exist in the first place, they will
accept the focus by default.

Task-number: QTBUG-39605
Change-Id: I74f01b85342df51aa3c0e80baf58ba1619c63438
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>