[TASK] Use name-resolution instead of strings where possible: 11

(part for EXT:backend)

Use name-resolution with ...::class instead of strings where
possible.
(needs PHP 5.5, which is meanwhile required for master)

Change-set Iedbb896607adddce2b23ab67f990fa2296c0d1bb
first introduced usage of ::class in master. We now add it
in various more places.

Change-Id: I49f47b7fb3dbee281af8e166afc3c36662730b54
Resolves: #63477
Releases: master
Reviewed-on: http://review.typo3.org/34867


Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>