Don't spin a nested event loop for loading schemas synchronously over HTTP.

Instead, use the internal synchronous load API in QNAM that relies on a
separate thread with a blocking queued connection, just like QtWebKit
does for synchronous XHR. FTP doesn't support this attribute, so we have
to keep the event loop trick for that one.

Change-Id: I785fa6d967160552b31778fcfc120d7ec4194a08
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>