GDA: multiple books

[Phil Longstaff]

The subject of multiple books has come up, especially with a GDA
backend.  qofsession.c has the comment:

    /* XXX why are we creating a book here? I think the books
    * need to be handled by the backend ... especially since
    * the backend may need to load multiple books ... XXX. FIXME.
    */

in routine qof_session_load().  I am not prepared at this point to
change the architecture to support book creation by the back end.  If
this is changed and a branch is provided, I will be happy to merge with
it and support multiple books.  Of course, GC would then need a new
backend API to query the set of books so that an appropriate UI could be
provided to the user.

Phil