Make QNetworkReply::downloadProgress() less chatty

For Qt 5.0, we should consider reducing the frequency with which downloadProgress() fires.

The problem is that it currently fires for every packet received from Layer 2, at least on desktop Linux. Which means that every 1000 bytes of so, an application's slot is (say) updating the progress bar, or updating internal variables about data downloaded. This sounds like excessive updates to the application code, particularly on mobile where every bit of CPU and graphics load is important. Additionally, on mobile devices, packets may arrive at the lower layers in sizes of a few hundreds bytes due to cellular data protocols.

Certainly, a workaround is for the application itself to throttle calling its slots/worker functions in response to a signal, but that would be a bit messy.