Use Javascript strict mode

src/webui/www/private/newcategory.html

@@ -9,6 +9,8 @@
     <script src="scripts/lib/mootools-1.2-more.js"></script>
     <script src="scripts/misc.js?locale=${LANG}"></script>
     <script>
+        'use strict';
+
         var newCategoryKeyboardEvents = new Keyboard({
             defaultEventType: 'keydown',
             events: {