[BUGFIX] Do not override hidden UC fields in user settings

While saving a user's uc record, all fields, which are set to disabled
in user TSconfig, are not touched anymore.
This is enforced by reducing the fields of showItem to the ones which
are allowed via user TSconfig.

Resolves: #77263
Releases: master, 7.6
Change-Id: Ib867efef731654373a3b1502c410ba5e182780e4
Reviewed-on: https://review.typo3.org/49224


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Martin Fox <kontakt@mlis.pl>
Tested-by: Martin Fox <kontakt@mlis.pl>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>