Add support for high-contrast mode
If the user activated a high-contrast mode then the app should use the system-provided high-contrast color scheme instead of a light/dark color scheme.
This MR is part two of the changes to address #2. It requires kguiaddons!170 (merged).
Marking as Draft until I verified that the system-QPalette-derived KColorScheme works.
I have verified this with the Gpg4win build of Kleopatra which uses the Positive* and Negative* colors from KColorScheme in a few places.
Edited by Ingo Klöcker