We migrated the part of the import to use DataHandler.
We didn't invest too much time as budgets are low.
Still the bugs are covered with tests and fixed.
Relates: #10782
A new PSR-14 event is added that allows to modify the categories to be
assigned to an event.
The event itself (including already existing categories) as well as the
list of categories to be used after import are available.
It is possible to change the categories to be assigned, e.g. keep some
of the existing categories.
That way it is possible for installations to add custom categories to
events.
Relates: #10623
