[TASK] Cleanup ext:backend

Cleanup ext:backend with the following code changes:
* Make use of the "use" statement for GeneralUtility
* Make use of the "use" statement for BackendUtility
* Case in switch statements are tabbed with one more tab

Change-Id: I6549316dddc9575cab4e661b7c111ec947757cb8
Fixes: #47900
Releases: 6.2
Reviewed-on: https://review.typo3.org/20479
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
[BUGFIX] Label in list view is not escaped

The label of a field in the list view is not escaped,
as the itemLabel function is broken, through the
third parameter which cannot be HSCed afterwards
if it contains HTML.

Change-Id: I5adcf0ce97dd9f5e8fd9546b367f55f1ac0c532e
Fixes: #29409
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/13817
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind