[TASK] Use name-resolution instead of strings where possible: 5

(part for EXT:form)

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: Ia97da7be3e8e0b0e21c1380620e0ce4d6450f1c0
Resolves: #63477
Releases: master
Reviewed-on: http://review.typo3.org/34858
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Tested-by: Alexander Opitz <opitz.alexander@googlemail.com>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>