Kirigami.Theme lacks buttonFocusColor from PlasmaCore.Theme

Kirigami.Theme is not a complete drop-in replacement for PlasmaCore.Theme. Turns out, it lacks QColor buttonFocusColor property.

You can find that identifier in Theme.qml files, but those are largely unused, and do not directly represent the Kirigami.Theme object that is being queried in client's code.

With that being said, the only usage of Kirigami.Theme.buttonFocusColor is in Kickoff -> Header.qml plasma/plasma-desktop!1574 (comment 725879)