[TASK] Deprecate t3editor->isEnabled()

Method isEnabled() always returns TRUE since the checked property
is protected and never reset to something else. It is unlikely that
an extending class changes this method. isEnabled() is deprecated now.

Resolves: #67171
Releases: master
Change-Id: Ia23163a3d36f797ea9e776663057592ce12f6a1f
Reviewed-on: http://review.typo3.org/39762


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>