QQmlXmlHttpRequest - use readyread signal not downloadProgress
QQmlXmlHttpRequest - use readyread signal not downloadProgress

The readyRead signal (part of QIODevice base class) is the
appropriate signal for knowing when there is data available
in a QNetworkReply.
DownloadProgress is intended for UI elements e.g. progress bars.
Previously it was emitted at the same time, but now it is emitted
less often.

Task-number: QTBUG-20449
Change-Id: I60abb8f807bde314785c301dac74d27238be1a23
Reviewed-by: Yunqiao Yin <25fb2aad3521dfc99c20c23480a10158b6555533@nokia.com>

jfaust/qtdeclarative,crobertd/qtdeclarative,qmlc/qtdeclarative,matthewvogt/qtdeclarative,qmlc/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,corngood/qtdeclarative,matthewvogt/qtdeclarative,jfaust/qtdeclarative,crobertd/qtdeclarative,jfaust/qtdeclarative,mgrunditz/qtdeclarative-2d,corngood/qtdeclarative,qmlc/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,corngood/qtdeclarative,mgrunditz/qtdeclarative-2d,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,crobertd/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,jfaust/qtdeclarative,corngood/qtdeclarative,qmlc/qtdeclarative,jfaust/qtdeclarative,corngood/qtdeclarative,corngood/qtdeclarative,matthewvogt/qtdeclarative,qmlc/qtdeclarative,crobertd/qtdeclarative,qmlc/qtdeclarative,qmlc/qtdeclarative,crobertd/qtdeclarative,crobertd/qtdeclarative,crobertd/qtdeclarative,corngood/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,jfaust/qtdeclarative,jfaust/qtdeclarative,matthewvogt/qtdeclarative