[BUGFIX] New values created by AddController are added correctly

New values created by AddController are now added again to the FlexForm
value by fixing the value parsing.

Additionally, the mixed up append/prepend values of `setValue` are fixed.

Resolves: #82049
Resolves: #82050
Releases: master, 8.7
Change-Id: Ifc2a4bf4ddd39a1b004db9945f832ca6cf9d765d
Reviewed-on: https://review.typo3.org/53646


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Sebastian Fischer <typo3@evoweb.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>