List items are now identified by an attribute, defined by the idAttribute option. By default
the id attribute is used.
The serialization format has changed to {expanded: [id...], collapsed: [id...], version: 2}.
The previous format returned by serialize() is also supported.
Unnecessary entries are excluded from serialization data structure (items without ids or children need not be included).
Checkbox ids are now sequential - if you want to address a checkbox use a selector that targets “.the-list-item > input”.
Remove the need for guid, generatedIdPrefix, specifiedIdPrefix and adding generated ids to list items.