[feature request] display cached thumbnails as themselves

This is perhaps a bit silly, but I figure it shouldn't be too hard to implement:

When viewing one of the thumbnail cache folders like $XDG_CACHE_HOME/thumbnails/normal, thumbnails are not generated for obvious circularity reasons. However, we could treat those files as their own thumbnails: when the normal sized thumbnail is requested for $XDG_CACHE_HOME/thumbnails/normal/hash.png, tumbler could simply return $XDG_CACHE_HOME/thumbnails/normal/hash.png.