[TASK] RTE: Optimize editor initialization

Ensure the CSS classes configuration is loaded while the editor is being constructed

Releases: master
Resolves: #63655
Change-Id: Iced9eeae16a0c9f66efb676702a8ec6893c1646c
Reviewed-on: http://review.typo3.org/35151
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>