QtIFW

Move the proxy factory base class into the downloader factory.

Remove some now superfluous code to silently try the system proxy
in error case, as now we have a proxy factory set on every downloader.

Change-Id: I3399ea473d2958dbf97d2f0048b60922eb98c74f
Reviewed-by: Niels Weber <niels.2.weber@nokia.com>
Reviewed-by: Tim Jenssen <tim.jenssen@nokia.com>