Renamed ObjectRegistry to ObjectWrapperManager, and replaced it's instance() method usage with assigning a ObjectWrapperManager

to the osgDB::Registry.  Added a osgDB::Registry::getObjectWrapperManager() for access of this object wrapper manager.  This
change centralises the singleton management in osgDB.

Merged the osgDB::GlobalLookUpTable functionality into ObjectWrapperManger to keep down the number of singletons in use.

src/osgDB/Registry.cpp

@@ -367,7 +367,9 @@ Registry::Registry()
     }
     
     // register http-protocol, so the curl can handle it, if necessary
-    registerProtocol("http"); 
+    registerProtocol("http");
+
+    _objectWrapperManager = new ObjectWrapperManager;
 }