[TASK] Focus text field if null placeholder checkbox is disabled

To improve the UX of null placeholder checkboxes (e.g. used in
sys_file_reference) its assigned text field is now automatically focused
if such checkbox gets disabled.

Resolves: #90879
Releases: master
Change-Id: I5228f49d58276d3c6fc1d19275ca9e92cbdb2f91
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64001


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Jonas Eberle <flightvision@googlemail.com>
Tested-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>