Avoid hardcoded field numbers in event query
[libcommhistory] Remove unused properties from Event

parentId is only relevant for SMS folders, which no longer exist.

Encoding, CharacterSet, and Language have no database representation or
use in this design.

isDeleted is not implemented and serves no purpose, but did have a
database field, which will need to be removed at some later point.
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.
Merge pull request #74 from special/messageparts

[libcommhistory] Implement message parts with SQLite