Draft: [kiconcolors] Add ButtonFocus color
Some of our icons (from breeze-icons) use ColorScheme-ButtonFocus for recoloring. This was introduced in https://phabricator.kde.org/D22647
However in KIconThemes/KIconColors we never actually fill it with a color. Only for the old plasma-framework-based icons we did that (https://invent.kde.org/plasma/libplasma/-/blob/kf5/src/plasma/private/theme_p.cpp?ref_type=heads#L611).
This is particularly noticeable for system tray icons like konv_message, which are now wrongly colored when using dark schemes: https://bugs.kde.org/show_bug.cgi?id=482645#c12
This patch makes sure that the appropriate color from KColorScheme is applied. However there is still a problem with system tray icons since they should be colored with the Plasma colors, not the application colors. This would need new API for KIconColors to pass the right color here