Added DockWidget::windowActiveChanged() signal

Tracking the top-level window's 'isActiveWindow' property is difficult
since the dock widget's top-level window is changing all the time
when docking and undocking. So added a convenience signal

src/DockWidgetBase.h

@@ -407,6 +407,11 @@ Q_SIGNALS:
     ///Only relevant for the auto-hide/sidebar feature
     void removedFromSideBar();
 
+    ///@brief Emitted when the top-level window this dock widget is in is activated or deactivated
+    ///This is convenience to replace tracking dockWidget->window(), since the window changes when
+    ///docking and undocking
+    void windowActiveChanged(bool activated);
+
 protected:
     void onParentChanged();
     void onShown(bool spontaneous);
@@ -478,6 +483,9 @@ private:
     ///@brief Updates the floatAction state
     void updateFloatAction();
 
+    ///@reimp
+    bool eventFilter(QObject *, QEvent *) override;
+
     class Private;
     Private *const d;
 };