Restart timeout timer when data is received in QgsNetworkAccessManager

When transferring large amounts of data via a slow connection, the transfer can time out even if data is still being transferred. This results in the client ending up with incomplete data.

This patch makes the timer restart as soon as any data is received (hence "timeout" is interpreted as the maximum time which can pass without any transfer activity). It also adds a warning message which is displayed to the user in case a transfer times out.

Funded by Sourcepole QGIS Enterprise.