Add registration of QQuickWebEngineDownloadItem / 1.7

Explicitly register the new property added in 7219986a

.
Because of QTBUG-50990 this is strictly speaking not necessary
at runtime, but makes sure that qmlplugindump also generates
metadata about the new property.

Change-Id: Idcc286aa7b25eb50d0f37cda8ee718d65a51e7f7
Reviewed-by: Michal Klocek <michal.klocek@qt.io>