[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)

src/qt/optionsmodel.h

@@ -7,6 +7,10 @@
 
 #include <QAbstractListModel>
 
+QT_BEGIN_NAMESPACE
+class QNetworkProxy;
+QT_END_NAMESPACE
+
 /** Interface from Qt to configuration data structure for Bitcoin client.
    To Qt, the options are presented as a list with the different options
    laid out vertically.
@@ -54,7 +58,7 @@ public:
     bool getMinimizeOnClose() { return fMinimizeOnClose; }
     int getDisplayUnit() { return nDisplayUnit; }
     bool getDisplayAddresses() { return bDisplayAddresses; }
-    bool getProxySettings(QString& proxyIP, quint16 &proxyPort) const;
+    bool getProxySettings(QNetworkProxy& proxy) const;
     bool getCoinControlFeatures() { return fCoinControlFeatures; }
     const QString& getOverriddenByCommandLine() { return strOverriddenByCommandLine; }