Added support for pre-seeding backups and adjusting source or backup locations when the on-disk data is being moved around.

Previously, if either source or destination location was changed in backup settings, the app would purge all delta copying data and the snapshot of destination if present. This caused destination to be re-scanned on the next run and all files to be re-copied in full next time they needed an update.

With the next relase (76.5) the app will now ask if the backup state should be purged or not. This way if you are setting up a remote backup job, you may first pre-seed the backup by attaching the drive locally and making a backup on it. They you'd take the device to the remote machine, change Destination path in the backup job, say No to the state reset and - voila - you are back in business as if the drive has always been remote.