[BUGFIX] Missing subcategories labels in EM

The (new) Extension Manager correctly uses subcategories for sorting
and grouping extension configuration options (in the configuration
form), but does not display the related labels. This makes the form
rather weird and unhelpful. This information must be restored.

Resolves: #44701
Releases: 6.0, 6.1
Change-Id: I0dde1d294edc403404ad39985f91e3772ef0b710
Reviewed-on: https://review.typo3.org/17660
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn