[TASK] Improve UX and accessibility of toolbar items

Several minor adjustments have been made to the toolbar items to
improve the UX and accessibility.

This includes, among other things, the following:
* Setting a correct heading hierarchy and adding some role attributes.
* When a bookmark has been assigned to a group, the section with the
  remaining ungrouped bookmarks now displays a corresponding heading.
* The form for editing bookmarks now has form labels and static input
  labels have been replaced.
* When no documents are currently open or have been used recently, the
  section headings are now displayed with corresponding info texts.
* Links to perform actions are now displayed in button style.

Resolves: #104129
Releases: main
Change-Id: I43030a078df4424f1a99c2020b347cfce498c949
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84738


Tested-by: Jasmina Ließmann <minapokhalo+typo3@gmail.com>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Jasmina Ließmann <minapokhalo+typo3@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>