Allow testing hiding the widget in the widgets sample.

This provides a convenient way for testing that it actually works, which is
currently not quite the case for wxSlider under OS X, see #2388.

samples/widgets/widgets.h

@@ -98,6 +98,7 @@ struct WidgetAttributes
         m_tooltip = "This is a tooltip";
 #endif // wxUSE_TOOLTIPS
         m_enabled = true;
+        m_show = true;
         m_dir = wxLayout_LeftToRight;
         m_variant = wxWINDOW_VARIANT_NORMAL;
         m_cursor = *wxSTANDARD_CURSOR;
@@ -114,6 +115,7 @@ struct WidgetAttributes
     wxColour m_colBg;
     wxColour m_colPageBg;
     bool m_enabled;
+    bool m_show;
     wxLayoutDirection m_dir;
     wxWindowVariant m_variant;
     wxCursor m_cursor;