In theory we support Qt 5.6 (because of SailfishOS) but in practice we are not checking that in the CI so probably we broke the compatibility long ago. Also, I don't think anyone is using this code in SailfishOS, so we can greatly simplify the code by requiring Qt 5.15 or later.
The `QElapsedTimer` was never started. Clean it up somewhat and use a dedicated
250ms timer for reporting transferred bytes periodically (like KIO does it) and
1000ms for calculating speed so it's not as erratic.
Furthermore, report final numbers in `slotResult` rather than complicating
`slotProcessedAmount` for that.
Also, fix typo "send" -> "sent"
* Set magic `destUrl` property to let it know the job destination in case of
multiple files being transferred
* Set a normal "Receiving File(s)" title like we do with "Copying" title
* Set device name and destination path as details
