feat: storing thumbnails on disk

[Rentib]

I don't know if this is completely outside the scope of the project, but I did a quick and dirty hack that caches thumbnails on disk similarly to how nsxiv does it. I ran some benchmarks and it saves 10 to 20 ms per image. The gallery still takes a very long time to load but the difference is there. If this is something that might get merged, I'd be happy to make a PR.