[BUGFIX] Fix "Select & upload files" button for FAL in flexform

Fix regular expression in escapeCssSelector method, to convert
e.g. "." to "\.".

Resolves: #83747
Branches: master, 8.7
Change-Id: I0fe19fb3590e28cafce6e2fd0cbf9a63f4375954
Reviewed-on: https://review.typo3.org/55516


Reviewed-by: Armin Vieweg <armin@v.ieweg.de>
Tested-by: Armin Vieweg <armin@v.ieweg.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>