QDeclarativeXmlHttpRequest - use readyread signal not downloadProgress
QDeclarativeXmlHttpRequest - 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: I732a2facb083f4cc35d01441a34964847f00fdb2
Reviewed-by: Yunqiao Yin <25fb2aad3521dfc99c20c23480a10158b6555533@nokia.com>

kobolabs/qtquick1,kobolabs/qtquick1,kobolabs/qtquick1,kobolabs/qtquick1,kobolabs/qtquick1,kobolabs/qtquick1