[BUG] RTE: backend layout of dialogues is broken

Releases: master
Resolves: #62789
Change-Id: I80b519ab85b7f18befdc451ce9558fccf68656cc
Reviewed-on: http://review.typo3.org/33911
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>