Deprecate add/remove_class.

From the widget roadmap: https://trello.com/b/5Xaj8Z7j/widgets
I'm not clear about the strength of discouragement you want. I would say there are two options:

1. Leave it as public API methods, but note in the docstrings that there are probably better things to do instead of using this (and describe what those are).
2. Make it private, and don't refer people to it in docstrings or warning messages. The only time we should tell someone to use a private API is if they come to us with a problem and that's the best way to solve it, and ideally we should then try to work out a better way to let them do what they want for the next release.

I think the important question is: if someone is using this at present, what should they be doing instead?