kcms/colors: Use selected link color from KColorScheme
QPalette has no such concept, but our styles, like Breeze, use it for links in selected items. Most noticeable by getting "orange" hyperlinks when selected (in the default Breeze colorscheme).
Color in the QML preview now matches the widget preview.
Technically it will be wrong for pure QStyles now I guess but at least in the default case (Breeze) it is proper now.