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/osgWrappers/osgDB/ImageOptions.cpp

@@ -38,7 +38,7 @@ END_REFLECTOR
 
 BEGIN_OBJECT_REFLECTOR(osgDB::ImageOptions)
 	I_DeclaringFile("osgDB/ImageOptions");
-	I_BaseType(osgDB::ReaderWriter::Options);
+	I_BaseType(osgDB::Options);
 	I_Constructor0(____ImageOptions,
 	               "",
 	               "");