Playlist Backup File Enhancement

[DSheirer]

See: https://forums.radioreference.com/threads/wipes-playlist-after-too-many-issues.478105/#post-4052354

Consider enhancing the playlist manager's playlist selection page to allow the user to see all of the backup files that exist and revert from one of those backup copies. Each of the copies should indicate the date and time when the backup was created and the size of the backup file. On selection of the backup file, the current version of the playlist should be backed up and the current version replaced with the content from the selected backup.

Tasks

1. Enhance the playlist manager to allow for multiple, timestamped backup copies of the playlist so that a user can select from multiple backup copies to rollback and ensure that a playlist doesn't get inadvertently deleted either through software errors or OS errors.
2. Enhance the User Preferences to allow the user to control the quantity of backup files.
3. Ensure the playlist manager cleans up any 'extra' backup copies that exceed the setting specified in the User Preferences.