[TASK] Move BackendController templates to one place

BackendController renders "Main.html" as central backend
skeleton, plus "Topbar.html" and "MainModule.html". The
patch moved the templates into one folder.

Resolves: #96404
Releases: main
Change-Id: I485d9ff03e457d78c8454e36ac893fbbc1ffe77c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72746


Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>