improve backup status screen for multiple keys

[tomholub]

• backups should be counted by longids, not by instances
• show three groups:
  1. properly backed up and imported keys: N
  2. backed-up but not imported: N [import missing backups]
  3. backup missing: N [backup keys]

[tomholub]

related to #956

[tomholub]

• backups should contain multiple keys in single file, all protected by the same pass phrase. It should
  be made clear which keys are getting backed up (but no need to let user choose)

Will create another issue