Use wxWindow instead of wxControl in the widgets sample.

Not all widgets are controls and we don't use any of wxControl-specific
methods in the sample, so don't require RecreateWidget() to return a wxControl
when a simple wxWindow suffices.

No real changes.

samples/widgets/spinbtn.cpp

@@ -86,7 +86,7 @@ public:
     SpinBtnWidgetsPage(WidgetsBookCtrl *book, wxImageList *imaglist);
     virtual ~SpinBtnWidgetsPage(){};
 
-    virtual wxControl *GetWidget() const wxOVERRIDE { return m_spinbtn; }
+    virtual wxWindow *GetWidget() const wxOVERRIDE { return m_spinbtn; }
     virtual Widgets GetWidgets() const wxOVERRIDE
     {
         Widgets widgets(WidgetsPage::GetWidgets());