Add support for Enums in Tracing

[Cécile Camillieri]

Currently the enumerations are not part of the Traceable elements, so they are not highlighted by the Tracing View.

To do this, REnum class has to implement Traceable, and the EnumViews have to register themselves with the GraphicalUpdater. Normally this will not require any changes to the weaver or the TracingView.

[Matthias Schoettle]

Maybe at the same time we can add support to message views for this, not sure exactly how though. It might be enough to add it for Message. It could also be enough to just look at operations...?!