Reconnect events to the widget after recreating it in the sample

This ensures that the expected events are given even after the widget is
recreated due to a change of some controls on its page (and not because
it's recreated by one of the menu commands in the parent frame).

We also don't need to connect to these events from WidgetsFrame any
longer (and, in fact, doing it would be wrong as we'd connect twice),
which simplifies its code.

samples/widgets/dirpicker.cpp

@@ -209,6 +209,8 @@ void DirPickerWidgetsPage::CreatePicker()
                                       wxGetHomeDir(), "Hello!",
                                       wxDefaultPosition, wxDefaultSize,
                                       style);
+
+    NotifyWidgetRecreation(m_dirPicker);
 }
 
 void DirPickerWidgetsPage::RecreatePicker()