New "Sign" and "Encrypt" icons for composer window

[schiessle]

Since Claws 3.16.0 which was released only a few days ago you can add a "encrypt" and "sign" toggle buttons to the toolbar of the composer window. At the moment this icons look out of place, probably because this icons didn't exists in earlier claws mail version.

Would be great if you could add some icons for this new actions to this awesome theme. 😃