Enable per network configuration proxy settings in QNetworkAccessManager

Enable per network configuration proxy settings in QNetworkAccessManager

Delayed the resolving of the proxy until the backend is being started.
This is because the proxy settings are not known until after
QNetworkAccessManager has brought the network online using
QNetworkSession.
On Nokia's symbian3 phones, the default network configuration is a
service network containing a list of access points in priority order.
For a typical user, this will include one or more WLAN networks and a
cellular network - each of which can have different proxy settings.

Task-number: QTBUG-18618
Reviewed-by: Peter Hartmann