[TASK] Clean-up H1-H4 tag hierarchy in backend

In backend headline tags should follow the semantic hierarchy.
Also for styling of headlines the hierarchy should be clear.

This patch brings semantically correct headline hierarchy for
all core backend modules and functionality: H1 through H4.

Also eases the migration path to include front-end css framework.

Change-Id: If91ec834098752ff4f7c30b0701cace578a2afd1
Fixes: #47785
Releases: 6.2
Reviewed-on: https://review.typo3.org/20373
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn