`reader.getFileSize()` return the size of the zim in Kbyte in a
`unsigned int` (32 bits). This is ok as it would overflow if the size
of the size is greater than 4294967295 kbytes (so ~4Tbytes).
However, we need to convert the return size into a unsigned 64 bits integer
else, when converting to bytes, we will overflow at 4Gbytes.
Even in `m_size` is a uint64_t.
When parsing a opds feed, the favicon is a url, not a dataurl.
If we download the favicon all the times, it may take a lot of time to
parse the feed.
We store the url and download the favicon only when needed (when displayed)