[BUGFIX] Checkboxes with itemProcFunc are not saved

Retrieve items added by an itemProcFunc before validating the current
set of selected checkboxes.

This fixes the bug that a item set consisting of mixed predefined and
dynamically added items always stored all predefined checkboxes as
selected (and no dynamic items).

It also fixes the case that a checkbox list consisting of purely
dynamically added items would never store a selection in the database,
showing all checkboxes as unselected.

Resolves: #76147
Releases: master, 7.6
Change-Id: I254a2936974f5bb5fbb6800a17667b66e1a86ca2
Reviewed-on: https://review.typo3.org/48082


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>