assembleModelInheritance modifies parent DecorationModel

After calling DefaultDecorationModelInheritanceAssembler.assembleModelInheritance, the parent DecorationModel is usually different from the original. In particular paths get relativized because MenuItems, Links, etc. get referenced instead of copied into the merged model. I think that only the child should be modified after the method returns, as otherwise it is impossible to resolve another child against the same parent. I am not sure if this has any practical consequences, as the DefaultSiteTool does not use the parent anymore after assemblance, but it could well be the source of some subtle bugs.