Feature: #86818 - Reintroduce keyboard accessible version of the pagetree

Description

This feature makes the pagetree focusable via keyboard using the tab key. Now it is also possible to use arrows, home and end keys in order to navigate through the pagetree. Besides that, using enter and space keys will open the page in the according content area.

Of course, it is still possible to use both mouse and keyboard navigation.

This change follows the best practices as described in WAI-ARIA Authoring Practices 1.1, see the W3 document for further reading.

Impact

Added tabindex, role, aria-* and id attributes to pagetree elements as advised in WAI-ARIA Authoring Practices 1.1. Screenreaders are now able to recognize the pagetree as tree element.