Allow switching tabs via context menu in the tabs area

This change allows you to switch tabs via the context menu. The context
menu only shows up if you click on the empty area beside the tabs.

Can be tested with `--allow-switch-tabs-via-menu` option in the example.

Fixes #232
Signed-off-by: Waqar Ahmed <waqar.ahmed@kdab.com>

src/private/widgets/TabWidgetWidget_p.h

@@ -65,6 +65,10 @@ protected:
     DockWidgetBase *dockwidgetAt(int index) const override;
     int currentIndex() const override;
 
+    /// @brief Shows the context menu. Override to implement your own context menu.
+    /// By default it's used to honour Config::Flag_AllowSwitchingTabsViaMenu
+    virtual void showContextMenu(QPoint pos);
+
 private:
     void updateMargins();
     void setupTabBarButtons();