ed. Selectable
       * will listen for this event from items and update the selection state.
       * Set to empty string to listen to no events.
       */
      activateEvent: {
        type: String,
        value: 'tap',
        observer: '_activateEventChanged'
      },

      /**
       * This is a CSS selector string.  If this is set, only items that match the CSS selector
       * are selectable.
       */
      selectable: String,

      /**
       * The class to set on elements when selected.
     