Split mapping.yaml into one file per backup

[mtkennerly]

What's your idea?

Right now, mapping.yaml has to keep track of the full sequence of backups. This causes some problems:

• If two computers have divergent backups, you either have to choose one set (using the cloud upload or download function) or manually edit mapping.yaml to combine them.
• You can't just copy/paste a backup in order to share it with someone.
• With automatic cloud sync enabled, if the local and cloud state are out of sync before a restore, all we can do is warn the user. We can't just download the latest cloud backups because we can't automatically resolve the conflicts in mapping.yaml.

By splitting mapping.yaml into one YAML file per backup, we can solve those issues:

• Divergent backups would be much less common. They would probably only happen when editing backup comments or locking a backup, which we'd need to make sure to sync.
• Full backups would be atomic and easy to copy. Differential backups would still need some care, though.
• We could download the latest backups more safely, unless an individual backup were present on both systems in a divergent state.