don't use archive names for extracted filenames

Instead, name:

- images like this: i0001.jpg, i0002.png, i0003.png, ...
- comment files like this: c0001.nfo, c0002.txt, c0003.nfo, ...
- sub-archives like this: a0001.cbz, a0002.cbr, a0003.pdf, ...

This way:

- we avoid potential file system encoding errors, invalid file names
- there is no possibility of "breaking" out of the temporary extraction directory
- we only have a flat directory, instead of an arbitrary tree