Implement DOMAttrModified event

[shogun70]

DOMAttrModified is supported on Firefox and Opera.
On IE it could be implemented by over-riding element.setAttribute() and a co-operating onpropertychange handler.
On Safari, etc it could be PARTIALLY supported by overriding setAttribute().

Speed would be a concern - ideally you could specify an attrName for the listener and the implementation could filter on that.
That wouldn't match the standard so we'll just have to leave it to XBLUI.