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/Output.cpp

@@ -63,7 +63,7 @@ void Output::init()
     }
 }
 
-void Output::setOptions(const ReaderWriter::Options* options)
+void Output::setOptions(const Options* options)
 {
     _options = options;
 }