[BUGFIX] Field is not added to TCA if part of fieldname exists

Explode current showitem and check if new field is already
available instead of using a wrong strpos check.

Resolves: #73490
Releases: master,7.6,6.2
Change-Id: If0269a1ee783ec8468ac3f7df915b671f17312ff
Reviewed-on: https://review.typo3.org/48663


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>