Use XML and other proper AS3 classes instead of the legacy XMLNode and such

• Acceptance Test - Add?:

  No

"The XMLNode class represents the legacy XML object that was present in ActionScript 2.0 and that was renamed in ActionScript 3.0. In ActionScript 3.0, consider using the new top-level XML class and related classes instead, which support E4X (ECMAScript for XML). The XMLNode class is present for backward compatibility."

XIFF uses heavily XMLNode.
Migrate to XML and XMLList.