[BUGFIX] Ensure correct sorting of extension configuration

If an extension uses a default subcategory for its configuration but does
not define a sorting order, using PHP >= 7 the order is reversed. This
patch ensures an previously introduced counter is used as identifier.

Resolves: #80180
Related: #76386
Releases: master, 7.6
Change-Id: Ib480632213ebe090688da5ba30c031ca116cf9a9
Reviewed-on: https://review.typo3.org/51980


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>