SQL backend status

[Phil Longstaff]

I've just committed a large change which:

1) splits the GDA backend into pieces, a generic SQL piece and a GDA specific piece
2) added a libdbi-specific piece which uses the same generic SQL piece (sqlite3 only)

I've currently disabled the gda-specific backend and have just the libdbi-specific piece.  'configure' does not yet check for the presence of the dbi headers, libraries and sqlite3 driver.  There is no new uri for the libdbi backend.  It uses 'file://' (or nothing).  Thus, for any file, gnucash will use the libdbi/sqlite3 backend if the file is an sqlite3 db, and will use the xml backend otherwise.  I haven't disabled the "Database Connection" menu entry, but could until I decide what I want to do with libgda.

I've done some basic testing, including loading and creating new db files.  As always, don't use for real data.

Phil