GStackWidget: Add a notification hook for when the active widget changes.

Libraries/LibGUI/GStackWidget.cpp

@@ -22,6 +22,8 @@ void GStackWidget::set_active_widget(GWidget* widget)
         m_active_widget->set_relative_rect(rect());
         m_active_widget->set_visible(true);
     }
+    if (on_active_widget_change)
+        on_active_widget_change(m_active_widget);
 }
 
 void GStackWidget::resize_event(GResizeEvent& event)

Libraries/LibGUI/GStackWidget.h

@@ -10,6 +10,8 @@ public:
     GWidget* active_widget() const { return m_active_widget; }
     void set_active_widget(GWidget*);
 
+    Function<void(GWidget*)> on_active_widget_change;
+
     virtual const char* class_name() const override { return "GStackWidget"; }
 
 protected: