Remove usage of TreeItem from MsgPartNetworkReply

REVIEW: 118641
Move processing of RFC2231-style string to a seperate method so it can be used in other places too
[gsoc] WIP
[gsoc][wip] first steps for new model
[gsoc][wip] new model can decrypt encrypted mails now
coding style: remove extra ;
[gsoc][wip] some progress towards displaying encrypted message parts
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
added support for encrypted message/rfc822 parts and fixed downloading of encrypted multiparts
fixed cmake to build without qca
cmake improvements and cleanup
create the proxy model asynchronously and handle that in the messageview and multipart widgets
get rid of that mutex and use Qt::QueuedConnection to gain asynchronous creation of parts
restructured and refactored the messagemodel
got basic decryption to work again
parse mime headers of LocalMessageParts and map dataChanged signals of ProxyMessageParts
moved the new model, added a common base class for OpenPGPHelper and SMIMEHelper, basic implementation of SMIMEHelper, fixed displaying of encryped messages and other stuff