[BUGFIX] Show hidden subgroups of fe_group

When editing a fe_group show its subgroups even if those are set to
hidden in order to avoid losing the subgroup relation on saving the
fe_group.

Resolves: #87649
Releases: master, 9.5
Change-Id: Ic39f32b47327202a848827bd9d6b4935c35edcdd
Reviewed-on: https://review.typo3.org/59640
Tested-by: TYPO3com <noreply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Josef Glatz <josef.glatz@typo3.org>
Tested-by: Josef Glatz <josef.glatz@typo3.org>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Tested-by: Andreas Wolf <andreas.wolf@typo3.org>

typo3/sysext/frontend/Configuration/TCA/fe_groups.php

@@ -54,7 +54,7 @@
                 'type' => 'select',
                 'renderType' => 'selectMultipleSideBySide',
                 'foreign_table' => 'fe_groups',
-                'foreign_table_where' => 'AND NOT(fe_groups.uid = ###THIS_UID###) AND fe_groups.hidden=0 ORDER BY fe_groups.title',
+                'foreign_table_where' => 'AND NOT(fe_groups.uid = ###THIS_UID###) ORDER BY fe_groups.title',
                 'enableMultiSelectFilterTextfield' => true,
                 'size' => 6,
                 'autoSizeMax' => 10,