[gui] Make proxy icon from statusbar clickable

Clicking on the proxy icon will open settings showing the network tab
Create enum Tab in OptionsModel
Use new connect syntax
Use lambda for private slots

src/qt/optionsdialog.h

@@ -40,8 +40,14 @@ public:
     explicit OptionsDialog(QWidget *parent, bool enableWallet);
     ~OptionsDialog();
 
+    enum Tab {
+        TAB_MAIN,
+        TAB_NETWORK,
+    };
+
     void setModel(OptionsModel *model);
     void setMapper();
+    void setCurrentTab(OptionsDialog::Tab tab);
 
 private Q_SLOTS:
     /* set OK button state (enabled / disabled) */