Backup enhancements

Hey there, for backup feature I recommend some enhancements:

• Incremental backup with some configuration settings (see also #2347 (closed))
  • do incremental backup every hour/day/week/month
  • do normal backup every day/week/month
  • save latest x full backups
• For hosting packages the possibility to define backup limits
  • no backups possible
  • incremental backups possible per hour/day/week/month
  • full backup possible per day/week/month
  • save maximum of x full backups
• Define external storage for backup
• ISPConfig backup server (possibility of multiple servers)