add support for display_name in categories

Summary:
categories description on ocs now supports a new tag, that is 'display_name'
which unlike 'name' is supposed to be user friendly and can be updated
by the server without clients actually breaking

display_name is not meant for matching purposes, only for displaying purposes because they correspond to the Category names used in the store.kde.org frontend.

for instance
https://api.kde-look.org/ocs/v1/content/categories

Test Plan: wallpaper ghns dialog uses display_name for displaying the category names now (it still uses the name-tag to match which categories are associated for the wallpaper ghns)

Reviewers: Plasma, leinir

Reviewed By: leinir

Subscribers: kossebau, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4120