add ability to delete unused wallpaper cache

[stardvst]

i've been trying a number of built-in wallpapers to choose one that i like.

upon changing between them, i noticed that previously unused wallpapers are being downloaded (or just loaded?).
so i suppose macos keeps a cache for already used wallpapers.

i run cleanmymac x to clean up this cache, but after the run i was still able to set wallpapers without macos first downloading them, which means macos is retrieving them from some cache.

i'm not sure if there is such cache, please let me know if you are aware of such. so i think it would be reasonable to add ability in cleanmymac x to remove this cache folder too upon cleanup.

thanks!