[BUGFIX] Correctly validate New Content Element entries

Each entry in the New Content Element wizard is tested to be valid,
i.e. the tt_content_defValues defined by that entry must be allowed
for the current BE user. Also, an entry may not select values that are
removed via TSconfig.

Setting TCEFORM.tt_content.[column].keepItems acts as a whitelist of
allowed values for *that* column. But the validity checks are done
hardcoded against the *CType* column.

Resolves: #23864
Releases: 6.2, 6.1
Change-Id: I5b6578f35552bd571fc822ac72f8f5fe12fe11f7
Reviewed-on: https://review.typo3.org/27588
Reviewed-by: Stefan Neufeind
Reviewed-by: Tymoteusz Motylewski
Tested-by: Tymoteusz Motylewski
Reviewed-by: Markus Klein
Tested-by: Markus Klein