Introduce the concept of layer set name, using the osg::Object::s/getName() to store

the setname, and using a compound string (set:setname:filename) in place of standard filename when reading and writing
files.

src/osgPlugins/ive/CompositeLayer.cpp

@@ -42,7 +42,7 @@ void CompositeLayer::write(DataOutputStream* out)
         else
         {
             out->writeBool(false);
-            out->writeString(getFileName(i));
+            out->writeString(getCompoundName(i));
         }
     }
 }