FIX: Compile Error due to Dependent Base Class Lookup Failure

This fixes a compile error due to a lookup error of a method in a template argument dependent base class. One of the more subtile things of C++ apparently. See this [StackOverflow entry](http://stackoverflow.com/questions/10639053/name-lookups-in-c-templates) for details.

TL;DR: `NotifyListeners()` is a member of the `Observable<>` template, which is dependant on the template parameter of `CatalogTraversal<T>` and thus cannot be inferred by the compiler without knowing the `T`.