Remove remaining pieces of old MessagePart loading code
[libcommhistory] Update MessagePart API
[libcommhistory] Store MessageParts data for events in sqlite

This is implemented similarly to extra properties. The message parts
will be queried after reading an event if a flag indicates that they
exist.

This does not store actual data, only metadata related to the parts
(including the path). Actual data storage and handling is left to users.
MessageParts role in EventModel