[BUGFIX] Fix keyboard handling of pagetree

The Treeview widget is only allowed to have one tabstop. Focus handling
within the widget is done using arrow keys.

When scrolling away from the focused node, and then focusing the widget
again, the focus is restored.

Resolves: #104061
Releases: main
Change-Id: If99bd5c5de506cbc649430fb8678736bdb7efdf6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84626


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: Benjamin Kott <benjamin.kott@outlook.com>