Split result object for signed and unsigned messages

Problem/Motivation

Since there are different properties for signed and unsigned messages, we should create different result classes.

Proposed resolution

Add:

• MailhandlerAnalyzerResultInterface with documented common methods for both signed and unsigned messages
• MailhandlerAnalyzerResultBase that implements common methods (sender, user, body)
• MailhandlerAnalyzerResultSigned with methods specific to signed messages (signature, signed text, PGP type)
• MailhandlerAnalyzerResult with methods specific to unsigned messages