qmlscene: don't call show() on the window if it's already showing

That way Component.onCompleted() can optionally do showMaximized()
without being overridden.

Task-number: QTBUG-31260
Change-Id: I6f5c3025efd3b38809be85da4172241695168a6c
Reviewed-by: Nikita Krupenko <krnekit@gmail.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>