qt3scan: add automatic backup of scans

[gadamc]

Sometimes an experimenter accidentally closes the application. Sometimes the application crashes. In both of these scenarios all data in the current scan are lost.

Add automatic backup of scan.
Backup should be in a known and configurable folder (eg $HOME/.qt3scan, dropbox or google drive type of folder that is continuously synched to the cloud, or write directly to s3-compatible bucket, hosted or local minio)
Backup filename should include timestamp
Backup after completion of every scan.
Backup when application is closed and scan was not saved (would put boolean flag in application that is set to False at the start of every new scan, and set to True when the user explicitly saves scan. Upon application closing, if the scan had not yet been saved, would automatically save scan to backup folder)
Attempt to backup before application crashes
Backup when user starts a new scan if they haven't saved the previous scan (again, would use the same mechanism as above with the boolean flag to indicate if scan was previously saved).

Also to consider -- in a separate thread, could probably implement continuous saving. As it would be occurring in a separate thread, wouldn't slow down scan.