Port away from KF5-only KColorSchemeManager::createSchemeSelectionMenu()
((stump branch in place of old commit))
This function is deprecated since KF 5.107 and gone in KF 6.
Investigating the needed changes showed also that the custom wrapper ColorSchemeChooser around the KF classes no longer is needed, instead the KF classes provide all the wanted config integration out-of-the-box.