Add functionality to sample backend

* set_properties
* save (message)
* delete_message
* create_attachments
* get_attachment_ids
It's not really the same. Each message in their parent's "message" list should have a singular MID. However, I think that we can open a message several times (or different people can access it at the same time). These messages live in the message cache, and therefore we can have several messages with the same MID there. That's how I understand it anyways, so please correct me if I'm wrong.