Preserve PrintTickets

[Ghostkeeper]

Print tickets are part of the core specifications. For a full implementation we'd need to read those and write them back. However Blender can do nothing with PrintTickets since it doesn't have any logic for how to 3D print anything. So we merely need to preserve the PrintTickets so that they get written down again when exporting to 3MF.

Since PrintTickets need to be associated with a model, they need to be stored in the metadata of the objects involved. Perhaps the metadata of a group.

Ideally we'd repair them if they are broken, to the best of our ability. However if that involves just deserialising to XML and serialising again when saving, that's fine too.