sort alphabetically category list

Summary:
sort the category list by their visible name
prefer sorting by displayName if available,
fallback to name otherwise

Test Plan: category dropdown still works and is sorted

Reviewers: Plasma, leinir

Reviewed By: leinir

Subscribers: broulik, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4156