KColorSchemeManager: don't override color scheme set by platform theme
This allows to have Qt platform theme setting the color scheme before it gets set and overriden by KColorSchemeManager. This is useful for 3rd party platform themes like QGnomePlatform, allowing them to set a color scheme that would complement QPalette. Since many KDE apps rely on using KColorScheme, it can happen that they will use a color from KColorScheme that doesn't correspond to QPalette set through the platform theme and end up being broken.