The new 'update_list_model_from_xml' function template simplifies the
use of the list model utility by alleviating the need for implementing a
custom policy class for each model. Instead, the transformation is done
using a few lambda functions given directly as arguments.
Issue #4317
The new 'apply_first' method enables users of the list model to manually
traverse the list model via the 'Element::next' method instead of
iterating via 'for_each'. This is needed in situations where the
list-model elements are visited via recursion, not via a loop.
Issue #3094
This patch makes the 'List_model' utility robust against duplicated
occurrences of node IDs in the supplied XML data. If two or more XML
nodes correspond to the same model element, the existing element is
updated with the information of the subsequent XML nodes.
