Use sidecar cover thumbnail files for AZW3 books

For some AZW3 files, the cover thumbnail is way off. It might show as either missing, or a small cropped image of something (but not the actual cover). Feature request to prefer a sidecar artwork file (any found .jpg or .png in fhe book folder) to use first. A fallback could be to try to use a cover from the embedded metadata, but this is very unreliable for .AZW3 files.