Segfault caused by setApplicationProxyFactory()

Program segfaults when the application-wide QNetworkProxyFactory set using setApplicationProxyFactory() is deleted due to either application exiting or calling setApplicationProxyFactory() again with a different QNetworkProxyFactory instance.

The segfault happens at `delete applicationLevelProxyFactory` in kernel/qnetworkproxy.cpp (both occurrences), which means pyside does not set the correct cpp wrapper (or atleast a deletable cpp wrapper) returned by pythonToCpp.

This means setApplicationProxyFactory() cannot be used at all, as it is certain to cause segfault.