[BUGFIX] Keep checked status of checkboxes

The checked status of checkboxes is stored in an array
(tx_powermail_pi1[field][marker][index]). The PrefillFieldViewHelper
will now check if an indexed value exists and keep the selected
checkboxes on validation errors.

Resolves: #56116
Releases: 2.1
Change-Id: I05406202ce9269a8423000e15c34e13f7d14c644
Reviewed-on: https://review.typo3.org/27716
Reviewed-by: Alex Kellner
Tested-by: Alex Kellner