[SlimDevices: Beta] Add time finished to Music Library Scan status report

[Phil Meyer]

>I really like this idea.
>
Sounds good, but be careful that we don't fill the disk up with old logs!

Could it keep the last <n> scan logs?  If fact, does anyone ever need to see historic scans?  Perhaps truncate the scan log file each time.

Alternatively, put some clear dividing line entry in the log file after a scan is complete.