Add isEmpty function to ItemList (#1218)

* Add isEmpty function to ItemList

* Fix coding style to be consistent.

* Recompiled app.js for both js/admin/ and js/forum/

js/lib/utils/ItemList.js

@@ -20,6 +20,22 @@ export default class ItemList {
     this.items = {};
   }
 
+  /**
+   * Check whether the list is empty.
+   *
+   * @returns {boolean}
+   * @public
+   */
+  isEmpty() {
+    for (const i in this.items) {
+      if(this.items.hasOwnProperty(i)) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
   /**
    * Check whether an item is present in the list.
    *