support state restore for drives with no-prescan set

[stevenhl]

It is possible to save state for folders on no-prescan drives, but the state will fail to restore unless the drive is manually scanned before the restore. This seems a bit non-intuitive. Perhaps, state restore should automatically scan the drive when the user invokes the restore. Since the user is requesting the action, the user will know which drives are available.

The current behavior which bypasses the no-prescan drives on the startup folder restore should be retained.

This behavior could be made a configuration option, it's not clear that this is a solution to the actual problem.