Validates if given element has a GXml.HashMap.attribute_key set, if so adds a new key pointing to given index and returns
true.
Attribute should be a valid DomElement attribute or a GXml.Object property identified using a nick with a '::' prefix.
If there are more elements with same key, they are kept as child nodes but the one in collection will be the last one to be found.
Return: false if element should not be added to collection.