ExecutableNode UI improvements

This pull request addresses the recently limited scope of #109, and clarifies (hopefully) the Despatcher and ExecutableNode classes with improved documentation.

I left out the change to doc/config/Doxyfile that John and I had discussed. I can add that here if you like, or one of us can just push it upstream.

I tried to squash a few more commits, but failed misserably due to rebase conflicts. The ones I would have squashed are pretty straightforward though, so hopefully it isn't too much to review as is.