Add metadata for backups to prevent full backups to be deleted too early

[MelanX]

Adds a file "metadata.json" for new backups. This is important if partial backups are enabled. This will delete the partial backups before deleting the full backups.
There are still some tweaks needed. I didn't check if everything works with full backups. For setting backupType = "MODIFIED_SINCE_LAST", there should be some improvements, too. The merge command needs to be adjusted, too. Here is a better overview:

• Check backupType = "FULL_BACKUPS"
• Tweak for backupType = "MODIFIED_SINCE_LAST"
• Implement in merge command
• Implement checking for manually deleted files on startup

Closes #39