Show categories in the UI

[nickvergessen]

Ref nextcloud/server#6868

The categories from info.xml should be displayed in the app store UI of an app:
https://apps.nextcloud.com/apps/admin_notifications

The people know in which category the app is displayed on the server

[tobiasKaminsky]

👍
Especially as there is no search in apps on server, one would have to go through all categories to find the desired app…