[BUGFIX] Prevent property mapping error in BackendUserGroup DTO

This adds a constructor to the BackendUserGroup DTO,
allowing the extbase ObjectConverter to determine the
correct target type of the "title" property.

Additionally, some cleanup in the DTO is done.

Resolves: #99708
Releases: main
Change-Id: I318e2ea30edee5f38e286ddf95603a983e681de2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77595


Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>