remove TreeItem from MsgPartNetworkReply so it can also work with the new model
improved handling of children to avoid multiple decryption of a message
added and improved some roles for the message model
use nullptr instead of NULL or 0
debug tree view for decrypted messages and error handling for decryption added
Move processing of RFC2231-style string to a seperate method so it can be used in other places too
added support for encrypted message/rfc822 parts and fixed downloading of encrypted multiparts