function getWorkingSet() {
        return _workingSet;
        // TODO: (issue #297) return a clone to prevent meddling?
    }
Returns a list of items in the working set in UI list order. May be 0-length, but never null.

When a file is added this list, DocumentManager dispatches a "workingSetAdd" event.
When a file is removed from list, DocumentManager dispatches a "workingSetRemove" event.
To listen for ALL changes to this list, you must listen for both events.

Which items belong in the working set is managed entirely by DocumentManager. Callers cannot
(yet) change this collection on their own.

@return {Array.<FileEntry>}