choke downloadProgress signals

choke downloadProgress signals

The QNetworkReply::downloadProgress signal is intended for updating
user interface components (e.g. a progress bar).
Limit signal emissions to 10 times per second, with an additional
signal just before the finished() signal to provide the 100% progress.

For the size of download where a progress bar is necessary, this
update frequency seems sufficient.

The implementation is done by dropping signals which would be emitted
less than 100ms after the previous signal emission.

Task-number: QTBUG-20449
Change-Id: I9c2dbe16c70f3270cbf98f3c74cf9d9a3f0ab900
Reviewed-by: David Faure <faure@kde.org>
Reviewed-by: Markus Goetz <markus@woboq.com>
Reviewed-by: Martin Petersson <Martin.Petersson@nokia.com>