Fix CompositeFileTransferJob speed calculation
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"