[TASK] Migrate topbar scaffolding to flexbox

This change migrates the responsive
topbar to flexbox, getting rid of some ugly
"position: fixed" functionality as well
as manual calculation of width / padding
via JS, making the scaffolding better to understand
and to use.

On top, this change fixes bugs related to styling
when using the topbar in non-desktop clients

Resolves: #93318
Releases: master
Change-Id: I510fd15cfce3243c0a8e6d5dcfcbd438476bb630
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/67465


Tested-by: Benjamin Franzke <bfr@qbus.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>