Secure storage using sailfish-secrets

https://github.com/sailfishos/sailfish-secrets: It was just announced on https://together.jolla.com/question/184471/release-notes-220-mouhijoki/ afaict.

It seems like a secure storage API, which might be worthwhile to use instead of the passphrase for storing the private key material.

Import from https://github.com/aebruno/whisperfish/issues/117