Improve thread safety and behavior of custom URL requests

Adds thread protection between the UI and IO threads access to the
QIODevice, and changes it to a QPointer, so we can tell if the user
deletes it.

Change-Id: I3e7b3f682d4c5a145d75cd5822affcfc9012cff7
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>