API inconsistency for Working Set vs. project tree decorators

[peterflynn]

It's potentially confusing that these APIs differ:

• Naming: addClassesProvider() vs. addClassProvider()
• The working set class provider can return null/undefined, while it's unclear if the project tree one can (though it can return "")
• The project tree icon provider can return a React.DOM.ins but the working set one cannot
• The project tree icon provider docs say it can return a raw DOM object, but the working set docs don't list this as allowed (I think it may work anyway though?)
• The working set icon provider docs say it can return undefined, but the project tree docs don't list this as allowed (and it appears that it would crash)

[peterflynn]

The situation is much improved with PR #9901: bullets 2, 4, and 5 are fixed. The remaining issues are just 1 and 3:

• Naming: addClassesProvider() vs. addClassProvider()
• The project tree icon provider can return a React.DOM.ins but the working set one cannot