[BUGFIX] TS/EM constant categories cannot be localized

Constant categories may now be localized using a construct similar
to defining custom subcategories:

Fixes: #57369
Documentation: #57371
Releases: 6.2
Change-Id: I6bea094d98897756772a6f616c8a2f3ab5fe7844
Reviewed-on: https://review.typo3.org/28903
Reviewed-by: Wouter Wolters
Reviewed-by: Fabien Udriot
Tested-by: Fabien Udriot
Tested-by: Wouter Wolters
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers