Show expected and detected algorithms

[yoshimo]

I have a few e-mails which are marked as "insecure signature algorithm". To be able to talk with the sender about this it would be nice to extend the tooltip a little and note what dkim verifier found what it would regard as secure instead.

[lieser]

Thanks for the suggestion. I will consider extending the tooltip, but unsure if it is a good idea, especially listing all considered good algorithms the addon supports (even if that is currently only one, rsa-sha256). In general I'm trying to not put to much technical details in the error messages, in the hope to make them more understandable to the average user.

Maybe the info would be better placed in an extended view (#160).