Wiki: explain how to use "_classes"
The wiki needs an explanation of how to use the core attribute "_classes". I recommend including it in the framework wiki [Developers Guide: Theme](https://github.com/adaptlearning/adapt_framework/wiki/Developers-Guide:-Theme) (it is an unfinished page). But it could go instead in the Vanilla wiki [Usage and Tips] (https://github.com/adaptlearning/adapt-contrib-vanilla/wiki/Usage-and-Tips). If done in Developers Guide: Theme, please put a link in Vanilla Usage and Tips to that topic (and vice versa).