Refactored the Registry::ReadFileCallback, WriteFileCallback and ReaderWriter::Options to they are now defined in their own header and in the osgDB namespace.

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

src/osgDB/XmlParser.cpp

@@ -18,7 +18,7 @@
 
 using namespace osgDB;
 
-XmlNode* osgDB::readXmlFile(const std::string& filename,const ReaderWriter::Options* options)
+XmlNode* osgDB::readXmlFile(const std::string& filename,const Options* options)
 {
     std::string foundFile = osgDB::findDataFile(filename, options);
     if (!foundFile.empty())