* Add a PinnedChildPages class to hold the list of child pages that should be displayed before the rest of the remaining child pages.
* Store the pinned top level pages on XWikiPreferences and the pinned child pages for a nested page on its WebPreferences.
* Add a wiki and space administration section to configure the pinned child pages.
* Implement a tree filter that excludes pinned child pages, so that they can be displayed first.
* Add support for grouping tree nodes and refactor the document tree node to use it.
* Implement a picker for pinned child pages (not yet connected to the REST resource, which needs to be implemented)
* Prepare the ground for refactoring the "Children" page REST resource to support the "reference" hierarchy mode, and for adding a wiki "Children" REST resource to get the top level pages.