Clean up FileTransferJob description

• 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

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This lets Plasma figure out where the files are going and offer useful actions when it finishes

Nicer details whilst transferring

"Open containing folder" action once finished transferring multiple files