Show thumbnail previews in a grid for confirming compression

It shows the file paths for now. Should be an opt-in feature with like a checkbox or something.Both [thumbnailer](https://docs.rs/thumbnailer/latest/thumbnailer/) and a relevant [rust-lang forum post](https://users.rust-lang.org/t/how-to-get-an-image-thumbnail-without-loading-whole-file-into-memory/79779) look pretty good for getting started.