Introduced a new FindFileCallback to Registry to compliement the existing ReadFileCallback and WriteFileCallback.
Added support for assign Find, Read and WriteFileCallbacks to osdDB::Options to enable plugins/applications to override the callbacks just for that
read/write call and any nested file operations
Converted Present3D across from using libxml2 to using the new osgDB::XmlNode/XmlNode::Input classes from Xml Parsing.
This changes removes the dependency on libxml2, and allows the present3D application and p3d to work on all platforms.