GDA: Porting to libgda 4

[Phil Longstaff]

The writing seems to be on the wall for the rev3 branch of libgda.
Their response to a bug report I filed against rev3 was that they
weren't going to be maintaining it but that I should shift to libgda
rev4.  I'm doing that now, when I can fit in time around my day job.  I
hope to have the port done so that we can do lots of testing before rev4
is released.

Phil