Separate "class" classes and "stateful" classes

[kitsonk]

Enhancement

Currently, this is no clear separation between classes that are statically bound and should be included in every render and classes that are "stateful" and should change based on the widget's state.

We should consider separating these clearly in widgets and using the array compostability features of dojo-compose to make it easy for a widget developer to allow classes to be mixed into the prototype for the widget.

Package Version: master