If and when we receive a double-click event sent by the CategoryTreeViewer,
hand on the event to the listener (typically the CitationDialog) so that it
can do something with the Category that the user has selected.
After loading a CiteBook, instead of having references to the Citations for this
Author, we will have a list of IDs, which we need to turn into real object references.
ReferenceGeneratorCollection: A collection of the ReferenceGenerators available to
the application, with routines to produce arrays, and to return a reference to
the user's chosen default reference generator.