GC architecture for backends

[Phil Longstaff]

I've started to look at what is needed to put a new backend into GC.  My
first questions is if this is documented anywhere.  I've needed to
update a few different files, and wonder if it couldn't be improved so
that each backend only needs to be known about in one place.  For
example, gnc_engine_init() in engine/gnc-engine.c needs to be modified
to ensure the backend module is loaded and registered with qof.  Then,
xaccResolveURLs() in engine/gnc-filepath-utils.c needs to be modified to
know about any new URI types (mysql://, sqlite://).  Seems to me that
after all of the backends are loaded, the engine should query all of the
providers so it would know about the URI types.

Phil