XWIKI-13468: The query used by the Document Tree to get the nested child pages is very costly

* Define a new Hibernate entity, XWikiPage, mapped to a SQL union between the documents table and the spaces table. This new entity is used in HQL queries to fetch the Nested Pages hierarchy.
* Separate the Document Tree code that queries the Nested Pages hierarchy from the code that displays the hierarchy. Move the former from Velocity to Java components. This way the Document Tree doesn't requrie PR for querying the Nested Pages hierarchy.
XWIKI-13615: "one more" does not make sense
* Query limit+1 child nodes and display only limit if there are more left.
XWIKI-12909: openTo doesn't work when root is specified as relative reference
XWIKI-13225: Finder in documentTree not working
* Resolve the relative root and openTo parameters.
XWIKI-13183: Some document tree branches not translated
* Display the translated page title
XWIKI-12832: Add support for sorting the pages by their title in the document tree
* Sort by the raw translated title field (with fall-back on the default title and then on page/space name)

(cherry picked from commit 8c642a7c)