Try to decrypt/verify later

Sometimes we can not decrypt or verify a mail because the the secret/public key is missing. But after importing a secret or public key we may can decrypt/verify previous received mails.

Add a function such that after importing a public key we try to

• decrypt mails which we are unable to decrypt (encState = EncryptionState.UnableToDecrypt variable exists in PersistentMail)
• verify mails which are signed but not correctly ( sigState = SignatureState.NoPublicKey variable exists in PersistentMail)

Fetching the mails must not be very efficient. We may change CoreData/Datahandler during the project.