This patch replaces the original policy-based 'update_from_xml' by a new
method that takes three functors for creating, destroying, and updating
elements as arguments. XML nodes are associated with their corresponding
internal data models by annotating the element type with the
'type_matches' class function and the 'matches' method.
The patch also improves safety by enforcing that list-model elements can
never be copied.
Fixes#4317