Bug 8802: On editing a library group category type is not set

Bug 8802: On editing a library group category type is not set

The category type was always set to 'searchdomain', because it's the
first of the dropdown list.

Test plan:
1/ Create or edit a library group
2/ Set the category type to "properties"
3/ Edit it again
4/ Confirm "properties" is correctly selected

Signed-off-by: Nick Clemens <nick@quecheelibrary.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>