Use wxUILocale for date formatting in wxDataViewDateRenderer

Use the appropriate format for the dates by using the UI, rather than C,
locale, similarly to wxGrid.

include/wx/dvrenderers.h

@@ -527,6 +527,8 @@ public:
     virtual wxSize GetSize() const wxOVERRIDE;
 
 private:
+    wxString FormatDate() const;
+
     wxDateTime    m_date;
 };
 #else // !wxUSE_DATEPICKCTRL