Port auto-encryption in KMail's composer to Trust Levels

It is a split out of !72 (closed). The parts that are have good test coverage and have only cosmetic GUI changes included the comments.

• auto-encryption code in KMail's composer to use the Trust Levels (from Kleo::trustLevel()) and adjusts the description strings accordingly. ( https://phabricator.kde.org/D4460)
• Use KeyResolverCore to determine best keys.
• Show directly in Compose if keys are near to expire not only after you want to send the mail.

Needs libkleo!38 (merged) to be merged to successfully run the tests.