We now have two ways to note that a URL has been added to a wallpaper via external means: urlDropped(KUrl) and setUrls(KUrl::List).
Drop urlDropped in favor of setUrls; make setUrls virtual, drop the protected addUrls slot (was just a work-around for BC) and port Wallpaper plugins away from addUrls. In WallpaperScript, make setUrls virtual and drop urlDropped.
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