[BUGFIX] Remember not rendered checkboxes in TCA treeSelect

When you have a select field of rendertype selectTree it looses the
selected values of the not rendered checkboxes.

This changes makes sure that also the not rendered values are kept.

Change-Id: I8649e83c56a0265a7de069ef9654ed13b90b3239
Resolves: #75519
Releases: master, 7.6, 6.2
Reviewed-on: https://review.typo3.org/47900


Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>