[TASK] RTE: Migrate editor iframe to plain JavaScript

Releases: master
Resolves: #63786
Change-Id: I3a01400a4790e173ed8e1d5410a708eedb23f63e
Reviewed-on: http://review.typo3.org/35332
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>