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/button.cpp

@@ -91,7 +91,7 @@ public:
     ButtonWidgetsPage(WidgetsBookCtrl *book, wxImageList *imaglist);
     virtual ~ButtonWidgetsPage(){};
 
-    virtual wxControl *GetWidget() const wxOVERRIDE { return m_button; }
+    virtual wxWindow *GetWidget() const wxOVERRIDE { return m_button; }
     virtual void RecreateWidget() wxOVERRIDE { CreateButton(); }
 
     // lazy creation of the content