Validates if given element has a attribute_primary_key and attribute_secondary_key set, if so adds a new keys
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 keys, 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.