[TASK] Make new toggle checkboxes keyboard accessible

Make the checkbox input field transparent, instead of hiding it completely,
so it can still be accessed via keyboard. 

Use the :focus CSS pseudo-class to highlight the input label 
with a box shadow, similar to current text input fields.

Resolves: #84936
Releases: master
Change-Id: I9e176cea1517617c978f05b63d881ab6af99380f
Reviewed-on: https://review.typo3.org/56871


Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>