configdict: T5837: add support to return added nodes when calling node_changed()
In the past, node_changed() suggested it would also return nodes that got added
(function comment) but in reality only deleted keys got accounted for.
This commit changes the signature and adds an argument expand_nodes to specify
the users interest of a node was deleted (default), added (expand_nodes=Diff.ADD)
or even both (expand_nodes=Diff.ADD|Diff.DELETE).