kcms/style: Add Union styles to Application Style KCM

Arjen Hiemstrarequested to merge
work/ahiemstra/unionstyles into master

Reason for the change

This integrates Union's styles into the Application Style KCM so that those styles can be selected as the system-wide application style. It also adds support for installing and uninstalling Union's styles to the KCM.

This is an alternative to !6897 (closed) that merges the data instead.

union!288 (merged) makes this work better.

Test plan

After building, with Union master, there should be at least an extra version of Union visible.

Screenshots or screen recordings

Screenshot_20260825_112810

Edited by Arjen Hiemstra

Merge request reports