function addClassProvider(callback) {
        if (!callback) {
            return;
        }
        _classProviders.push(callback);
        // build all views so the provider has a chance to style
        //    all items that have already been created
        refresh(true);
    }
Adds a CSS class provider, invoked before each working set item is created or updated. When called
to update an existing item, all previously applied classes have been cleared.

@param {!function(!{name:string, fullPath:string, isFile:boolean}):?string} callback
Return a string containing space-separated CSS class(es) to add, or undefined to leave CSS unchanged.