DockPanel.Persistor / SaveAsXml: Use XmlWriter.Create

Using 'XmlWriter.Create' prevent the underlying stream from beeing closed when the XmlWriter ist closed. For the FileStream used in the source file, isnt' a bug because the file is written well. But if you're like to write the output to a MemomryStream, you can't read the result, if the (underlying) MemoryStream has been closed.