desktop: Do not overwrite wallpaper-provided accent color

Original commit message(s)

• desktop: Do not overwrite wallpaper-provided accent color

Reason for the change

When a wallpaper provides an explicit accent color, Desktop.qml assigns it to the desktop. However, the following independent if statement immediately replaces it with the dominant color calculated from the wallpaper image.

Test plan

Use a valid image wallpaper package with the following metadata.json:

{
    "KPlugin": {
        "Id": "org.kde.test.wallpaperaccent",
        "Name": "Wallpaper Accent Test",
        "License": "CC0-1.0"
    },
    "X-KDE-PlasmaImageWallpaper-AccentColor": {
        "Light": "#3d818d",
        "Dark": "#018786"
    }
}

1. Enable the option to derive the accent color from the current wallpaper.
2. Select this wallpaper package.
3. Use a light color scheme and verify that the accent color (#3d818d).
4. Use a dark color scheme and verify that the accent color (#018786).

Screenshots or screen recordings

Before	After

Bugs fixed

BUG: 514656