Provide an option wheether to use the system proxy settings

Authored by marten on Jun 19 2019, 6:21 AM.

Description

Provide an option wheether to use the system proxy settings

This is not normally useful, so do not use the proxy by default.
Added function Session::setUseNetworkProxy() to use the proxy
if required.

Differential Revision: https://phabricator.kde.org/D21463