wallpapers/image: support setting custom accent color for dark variant

Also add a test

Example:

Light	Dark

{
    "KPlugin": {
        "Id": "TestTest",
        "License": "CC-BY-SA-4.0",
        "Name": "Test"
    },
    "X-KDE-PlasmaImageWallpaper-AccentColor": {
        "Light": "red",
        "Dark": "cyan"
    }
}