Comic Book - Support more image formats for thumbnail generation

The current thumbnailer for .cbz, .cb7, .cbt, etc. Doesn't support many image formats aside from the more common ones

I usualy use this format for archiving large photo libraries into a single file after compressing them into formats like AVIF, and I noticed that dolphin clearly lacked the ability to generate thumbnails when the "cover" was in .avif format

The change adds support for other image formats that, while useful, tend to be forgotten most of the time