[FEATURE] Get flexform data in getText()

It is now possible to access the content of a flexform property
in TypoScript.

Resolves: #17309
Releases: master
Change-Id: I80c68597908fc16083c52dc135abbebf94179e4c
Reviewed-on: https://review.typo3.org/49639


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>