[BUGFIX] RTE: Broken rendering of dropdowns in toolbar and dialogues

Releases: master
Resolves: #63578
Change-Id: Iea92c932d56dbfabcea7f884b3ebb3f8b0f218ce
Reviewed-on: http://review.typo3.org/35057
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>