[FIX] Correctly apply rules for signed or encrypted messages [#203 state:fixed]

- The rules are applied everytime an encrypted|signed message is done
  processing.
- Signed messages are only processed once per session, encrypted ones
  everytime, since they might contain signed messages, but the key might
  not yet be in cache, so it would not be possible, to check the decrypted content.
- Rules are applied as soon as possible, but the latest, once the user actively selects
  a message. This restriction is due to the fact, that in order to check if messages
  are encrypted & signed, the messages needs to be decrypted and that requires
  the user's passphrase, which they are only prompted for, when the message is actively
  selected, unless it's already in the cache.