[FEATURE] EXT:form - Add the "Confirmation" finisher to the form editor

Make the "Confirmation message" finisher available within the form editor.

The BE-editor can choose between 2 possibilities:

* Enter the confirmation message (plain text) into a textarea.
* Select a content element that is rendered instead.

Resolves: #80187
Releases: master
Change-Id: Ic163964669c95c98a38726f5d814b696d6e7f7bc
Reviewed-on: https://review.typo3.org/52007


Tested-by: TYPO3com <no-reply@typo3.com>
Tested-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Reviewed-by: Martin Kutschker <martin.kutschker@ymail.com>
Tested-by: Martin Kutschker <martin.kutschker@ymail.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>