Modernize EBookBackend's public API.

Modernize EBookBackend's public API.

This leaves EBookBackend and EDataBook engaging in a somewhat ridiculous
kind of ping-pong game for the moment, but the desired public API (in as
much as a backend has a public API) is starting to take shape.

With the scaffolding now in place, we can begin converting backends to
use modern GIO conventions one method at a time, and eventually remove
all the "respond" functions in EDataBook.