[BUGFIX] JS: Fix FormEngine initialization

The FormEngine initialization process needs to be very careful
when the DOM is accessed.
This patch separates the routines and encapsulates those in
a DOMready handler, which are critical.

This solves a possible race condition when JS is executed faster
than DOM is built.

Releases: master, 7.6
Resolves: #80481
Resolves: #80366
Change-Id: I205aebc9f87a25f06942f923497f7f535fdb0c8f
Reviewed-on: https://review.typo3.org/52180


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Thomas Maroschik <tmaroschik@dfau.de>
Tested-by: Thomas Maroschik <tmaroschik@dfau.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>