minor cleanup/restructure of how backends are located and loaded.

[Linas Vepstas]

Log Message:
-----------
minor cleanup/restructure of how backends are located and loaded.
This infrastructure is not currently used by GnuCash.

Modified Files:
--------------
    gnucash/src/engine:
        qofbackend-p.h
        qofbackend.h
        qofsession.c