Add support for anonymous mode

This mode was added in libtorrent v0.16 and should be used in
addition to a SOCKS5 proxy.

src/preferences/options_imp.h

@@ -84,6 +84,7 @@ private slots:
   void on_btnWebUiKey_clicked();
   void on_registerDNSBtn_clicked();
   void setLocale(const QString &locale);
+  void toggleAnonymousMode(bool enabled);
 
 private:
   // Methods