[BUGFIX] Override flex form field label with page TSConfig

Move the data handling to calculate a final field label
value from PaletteAndSingleFieldContainer to an own data
provider and register this provider for full database records
and for flex form processing.
This way flex form field label overrides via page TSConfig are fixed,
eg. this changes the "Order By" label of ext:news flex form:

TCEFORM.tt_content.pi_flexform.news_pi1.sDEF.settings\.orderBy.label = override

Language specific overrides in flex fields now work as well

...settings\.orderBy.label.fr = French override

Change-Id: I02474e9cca9db3e949c217f21f5704ec16641545
Resolves: #75606
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/47919


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Stefan Froemken <froemken@gmail.com>
Tested-by: Stefan Froemken <froemken@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>