[BUGFIX] Add placeholder to date picker form elements

Add the possibility to define a placeholder html attribute for
date picker form elements.

Resolves: #82514
Releases: master, 8.7
Change-Id: I7bf272d4e1e9b0af1c15e6ceeb6e1b91afba327e
Reviewed-on: https://review.typo3.org/55835


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>