Rationale behind this PR:

Many torrents contains several playable files (series, albums ...).
Good players like VLC has posibility to open a playlist - combined with streaming capability of rqbit this enables to play whole content of a torrent in the player (just by open playlist link in the player).

This is a minimal implementation, that works with VLC, couple more things I thought about:

• MIME type of playlist - the only official MIME is registered by Apple for HLS - this is not exactly this case as as text/plain works with VLC, so I did not explore further - this article is good source for m3u playlist "standard" - https://en.wikipedia.org/wiki/M3U#Internet_media_types
• ordering of files - now using the ordering provided by the program, not sure if reodering could be useful.
• UI icon - now on whole torrent - but I though also on possibility to have it on directory level. Current implementation was easiest to implement. But possibility to have playlist for just particular directory sounds interesting.