Is it conventional to provide a list of GUI element descriptions in a thesis?
I think you are writing a thesis, not a user manual for a program. Focus on the technical advances. That is what your thesis will be judged on.
I'd suggest leaving the GUI out of it unless you have designed new, and innovative, human-computer interaction elements. Creating such elements has value, certainly, but it doesn't sound like that is the focus of your work.
If you want a user manual, make that a separate project - just for fun.