Implement hierarchical XML element access (#1338).

Hierarchical element access methods have been added to GXml and GXmlNode.Using a string in the format "params > param[1] > value > struct" one can in a single method call walk down the hierarchy of an XML document to access an element. The squared brackets allow specifying an element index in case that multiple elements with the same name exist. If squared brackets are omitted, an index of 0 is assumed. If the indicated element does not exist, the method returns a pointer to NULL.