Moved Xom to its own library libxom

  Xom is a library dependent of GXml to serialize GObjects to XML,
  providing a .NET Serialization like framework with a flexible,
  overriadable implementation. Xom.Serializable interface allows
  to create your own implementations. Xom provides serializable
  Gee collections to store a set of Serializabe implementors
  objects and serialize them to a XML document or an XML Element.