function getAllOpenDocuments() {
        var result = [];
        var id;
        for (id in _openDocuments) {
            if (_openDocuments.hasOwnProperty(id)) {
                result.push(_openDocuments[id]);
            }
        }
        return result;
    }
Returns all Documents that are 'open' in the UI somewhere (for now, this means open in an
inline editor and/or a full-size editor). Only these Documents can be modified, and only
these Documents are synced with external changes on disk.
@return {Array.<Document>}