[FEATURE] EXT:form - Add element selector for text editors

Adds a new button to the text editors.
This splitbutton opens a list with available form elements.
If a form element is choosen, the form element identifier will be
inserted into the property text editor.

Resolves: #79442
Releases: master
Change-Id: If40114958311200348bde220f6cf62fb6365260b
Reviewed-on: https://review.typo3.org/51424


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Andreas Steiger <typo3@andreassteiger.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>