Use Qt API instead of KColorSchemeWatcher

Nicolas Fellarequested to merge
work/nico/no-watcher into master

Qt has all the API we need, at least since Qt 6.10.

We cannot depend on Qt 6.10 yet though. The only thing that KColorSchemeManager does that isn't available in Qt < 6.10 is support for detecting high-contrast mode on Windows. Since that is only a little bit of code we can include that here

Edited by Nicolas Fella

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