Deprecate add/remove_class.

From the widget roadmap: https://trello.com/b/5Xaj8Z7j/widgets
> I think the important question is: if someone is using this at present, what should they be doing instead?

There is no alternative.  I think there are two reasons for deprecating these methods: 
- We are trying to abstract "browserisms".
- The path dependent nature of of add/remove_class leaves something to be desired.  It's annoying that they only work after the widget has been displayed.