Use color scheme based on OS system dark/light mode preferences

[christianparpart]

Closes #604

Checklist

• get dark/light theme mode events on Linux
• get dark/light theme mode events on MacOS
• get dark/light theme mode events on Windows (how)
• config: extend config to allow specifying dark and light mode themes (then the system setting will decide what to chose from) or just one and then this is always used
• apply color scheme based on that.
• propagate dark/light mode changes to the app (VT extension to query state and get realtime notifications on state changes)
• reflect changes in documentation
• changelog entry added to metainfo.xml

2023-10-07.23-02-14.mp4

[Yaraslaut]

Documentation is missing :)

[christianparpart]

Documentation is missing :)

@Yaraslaut I sadly didn't decide yet how the configuration should look like. What you see is my proposal. But if you have an idea on how it could be done better, I'm absolutely open - then of course we must document that within this PR as well :)

[christianparpart]

MacOS bits as provided by @uspasojevic96:

#import <Cocoa/Cocoa.h>
- (BOOL)isDarkMode {
    NSAppearance *currentAppearance = [NSAppearance currentAppearance];
    if (currentAppearance) {
        // Check if the current appearance is dark
        if (@available(macOS 10.14, *)) {
            return [currentAppearance bestMatchFromAppearancesWithNames:@[NSAppearanceNameDarkAqua]] == NSAppearanceNameDarkAqua;
        }
    }
    
    // Default to light mode if appearance cannot be determined
    return NO;
}

For event subscription:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appearanceDidChange:) name:NSAppearanceDidChangeNotification object:nil];

I will probably need some help to clue that into the code base, as I've never touched ObjC/++ just yet :|