Add a setting to clear the values of proxy system properties

With a VPN configuration clearing the routes and setting system properties for proxy servers, socket connections end up trying to go through a socks server and, depending on how that one is configured, may hung there until it times out.

Ignoring the values put in system properties re-enable direct socket connection and this pull-request introduces a setting in "Proxy Settings" to select that if desired.

Note that this is still required even if the new proxy setting is set to None in preferences :  the jvm uses a SocksSocketImpl by default which looks for socksProxyHost and socksProxyPort and try to connect to a socks server if they are defined.