Provide instance()-method for Utils::NetworkAccessManager.

Preemptively fix exit warning "QEventLoop cannot be used
without QApplication" (Qt 5/dev) emitted by destruction of
the of the generic highlighter manager singleton which had a
member of type Utils::NetworkAccessManager.

Use a single instance of Utils::NetworkAccessManager for the
main thread and clean up properly using a qAddPostRoutine().

Change-Id: Ida57b9028a79eb4927818ce49088ea567f3bdfd6
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/cpaster/urlopenprotocol.cpp

@@ -35,8 +35,8 @@
 
 using namespace CodePaster;
 
-UrlOpenProtocol::UrlOpenProtocol(const NetworkAccessManagerProxyPtr &nw)
-    : NetworkProtocol(nw), m_fetchReply(0)
+UrlOpenProtocol::UrlOpenProtocol()
+    : m_fetchReply(0)
 {
 }