NotificationArea = function (selector, options) {
        this.selector = selector;
        this.events = options.events;
        if (this.selector !== undefined) {
            this.element = $(selector);
        }
        this.widget_dict = {};
    }
Construct the NotificationArea object. Options are:
    events: $(Events) instance
    save_widget: SaveWidget instance
    notebook: Notebook instance
    keyboard_manager: KeyboardManager instance

@constructor
@param {string} selector - a jQuery selector string for the
notification area element
@param {Object} [options] - a dictionary of keyword arguments.