Make getElementsByClass() work without given tag

This fixes the hiding of the highlighting of the current section when
the mouse leaves the edit section button.

lib/scripts/compatibility.js

@@ -99,6 +99,7 @@ function findPosY(object){
 function getElementsByClass(searchClass,node,tag){
     DEPRECATED('Use jQuery() instead');
     if(node == null) node = document;
+    if(typeof tag === 'undefined') tag = '';
     return jQuery(node).find(tag+'.'+searchClass).toArray();
 }