Fix loading nested mpegurls using GStreamer

If gstreamer tries to load an http source, we currently lose the context
of where it is loaded from, which causes the load to fail.

This patch caches a loading context when one is set on the outer most
appsrc, and uses that for gstreamer requests until the outer most source
is unloaded.

This can have consequences if multiple webviews are open at the same
time with different QNetworkAccessManagers, but this is considered
unusual, and wouldn't work with the QtMultimedia either.

Task-number: QTBUG-37636
Change-Id: I9d3b4d10705254629bc5fce7d76765077585caa2
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>