example: Add an option to show a non-closable dock widget

Passing -n to the example will make the dock widget #0 non-closable

examples/dockwidgets/MyMainWindow.h

@@ -27,6 +27,7 @@ class MyMainWindow : public KDDockWidgets::MainWindow
     Q_OBJECT
 public:
     explicit MyMainWindow(const QString &uniqueName, KDDockWidgets::MainWindowOptions options,
+                          bool dockWidget0IsNonClosable,
                           const QString &affinityName = {}, // Usually not needed. Just here to show the feature.
                           QWidget *parent = nullptr);
 
@@ -34,4 +35,5 @@ private:
     void createDockWidgets();
     KDDockWidgets::DockWidgetBase* newDockWidget();
     QMenu *m_toggleMenu = nullptr;
+    const bool m_dockWidget0IsNonClosable;
 };