Implement colorScheme for Qt::ColorScheme::Dark and ::Light
Qt's KDE theme has a heuristic for determining the color scheme dark or light preference based on the theme name and palette. This code path was not used when plasma-integration was loaded instead.
This implements colorScheme based on how we do it in xdg-desktop-portal-kde based on the window color.
Application.styleHints.colorScheme
is now populated correctly and updates as I change the color scheme.
Should this go in 6.0?