Feature: Force file check

[MayeulC]

Most clients offer a way to re-check existing files. It allows recovering from mistakes like deleting or overwriting part of a file, detecting silent corruption and getting part of the file from another source.

In my case, I just wanted to transfer a torrent to simple-torrent so that it would start seeding it. That initial transfer was exceedingly slow, so I just copied the file over. However, progress remains at a few percent, and there is no way to force checking the file.

Force checking should check both the temporary folder, and the destination folder.