[BUGFIX] Define "label" for tt_content form elements within form setup

Define the "label" property for tt_content form elements within
the form setup to allow saving tt_content form elements.

Resolves: #85578
Releases: master, 8.7
Change-Id: I490a43692a9f54ef843e9362d0f700a9e5552106
Reviewed-on: https://review.typo3.org/57616


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: Mathias Brodala <mbrodala@pagemachine.de>