Gui: Add support for asynchronous loading of the structure of encrypted and signed message parts
 MessageModel and MessagePartFactory added

 The new Model is a proxy model for the Imap::Mailbox::Model. It provides
 a way to insert MessageParts into a message that are not known to the
 IMAP server. This is usefull for displaying decrypted data from encryped
 messages.
Gui: Add a debug widget for displaying the MIME structure or a message

This is useful as the message structure visible to the MessageView
can be different from the structure known to the MailboxTree.
Tests: Test for the MessageModel added
Support for decrypting PGP/MIME messages added

REVIEW: 117363