[BUGFIX] Initialize T3editor properly

If a T3editor is instantiated on-the-fly, it is not initialized properly.
Add a callback to the ``Form/Element/T3editorElement`` class and mark
editor instances as initialized to prevent double initialization.

Change-Id: I8c6b692eb2c132180ed0d122b35515276df5acd9
Resolves: #73425
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/46680


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>