[BUGFIX] Recognize changes in FlexForm section containers

If section containers are changed (i.e. new containers are added or
existing ones are deleted), the change is now recognized by adding a
class `has-change` to the appropriate container. This assures the
modal on exit is shown to the backend user to avoid discarding
onsaved changes.

Resolves: #95290
Releases: main, 11.5
Change-Id: Iff85a98f5f93876d1151909d28641ac6cca70874
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73706


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>