Provide a world backup button on world selection screen

Since we already have the functionality to backup a world when upgrading versions or using a snapshot version, it should be relatively easy to provide a simple button in the world selection screen to also run the same functionality on demand.

We don't need to have a download or export function where we choose to back it up, it can simply backup to where backups are already stored, and then the user can do this whenever they feel like it, without having to copy files via the operating system. It would be a nice to have enhancement for users.