Straighten up file transfer cancellation

There was a mess with fileTransferCancelled(); it was only emitted when
a download (but not an upload) was cancelled; besides, in case of
downloads a file transfer info structure was getting deleted whereas
uploads left a file transfer in Cancelled status. This all now converges
on:
- fileTransferFailed() for both failures and cancellations (to simplify
  slot connection, and also to follow the practice in, e.g., Qt Network).
- the file transfer info structure is kept around in Cancelled status,
  following the logic used for failures. There's no particular cleanup
  which may become a problem if one uploads and cancels many times
  (download file transfers are keyed to event ids, mitigating
  the problem); this will be fixed in another commit.

Closes #503. Closes #504.

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