desk.cpl: The current wallpaper is always querried from "Wallpaper"

When the applet queries the users current wallpaper, it always uses the content of the "Wallpaper" value.

The patch adds a few lines to use "ConvertedWallpaper" if "Wallpaper" and "OriginalWallpaper" have
the same content. Otherwise "Wallpaper" is used.