[BUGFIX] RTE: Button not disabled when in text mode

Releases: master
Resolves: #63983
Change-Id: Idffdd3facd8e7cf9c0f4de9f7f2df7d344ba2b94
Reviewed-on: http://review.typo3.org/35575
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>