Add DockWidgetBase::isFocusedChanged

Fixes: #60

src/DockWidgetBase.h

@@ -318,6 +318,7 @@ public:
     ///@brief Returns whether This or any child of this dock widget is focused
     ///Not to be confused with QWidget::hasFocus(), which just refers to 1 widget. This includes
     ///variant includes children.
+    ///@sa isFocusedChanged()
     bool isFocused() const;
 
 Q_SIGNALS:
@@ -343,6 +344,10 @@ Q_SIGNALS:
     ///@sa setOptions(), options()
     void optionsChanged(KDDockWidgets::DockWidgetBase::Options);
 
+    ///@brief emitted when isFocused changes
+    ///@sa isFocused
+    void isFocusedChanged(bool);
+
 protected:
     void onParentChanged();
     void onShown(bool spontaneous);