Draft: Use in-place modification API of KLocalizedString
Makes use of new API proposed in frameworks/ki18n!36 (closed), so right now usage example.
Affects only signature of private method of public class, so should not need any version bumping for ABI/API reasons.
Also not using any KI18N version code variants, IMHO merging can be delayed until KF 5.90 is official requirement (or whenever the new API is in)