Add functionality to sample backend

* set_properties
* save (message)
* delete_message
* create_attachments
* get_attachment_ids
An argument of move/copy is a list with the MIDs of the copied messages. These MIDs are meant to be new ones, since a MID is unique across the system.

Both oxcfold.c and the Python bindings take care of getting new IDs for the copied messages in the target folder.