[Qt] let OptionsModel::getProxySettings() directly query proxy

• as a proxy set via GUI can be overridden via -proxy, directly query the core to get active proxy
• give a warning, if active proxy is not SOCKS5 (needs to be SOCKS5 for the Qt networking code to work)
• also remove an obsolete connect() call from optionsdialog.cpp and a reference to Bitcoin-Qt (now just GUI)