To be honest I have never seen such a way to transfer parameters but KMime and our new MIMETreeparser in T6199 can probably handle them but our old and trusty RFC822parse code in GpgOL needs to be adjusted.

Current progress, the viewer now support viewing embedded calendar events, embedded emails and embedded calendar events in an embedded email

Current progress left being the qml version for Kalendar and right the QtWidget version for Kleopatra. This still require some work in the QtWidget viewer to display that the message is signed and encrypted

Yes, this might make sense to have a library for the core parsing / reparsing part and then maybe some widgets in Libkleo that could be used to show the information. So that we could in theory create a small executable that just instantiates a libkleo widget to show a MIME mail and Kleopatra could use that then, too. But for clients like Kalendar which don't necessarily use QWidgets it is porbably better to have this split out.

I started working on it. Current progress, I managed to move the mimetreeparser/partmodel from kalendar to libkleo and removed the few akonadi bits.