Change to libgdamm-6.0

* configure.ac: Use libgda-6.0 instead of libgda-5.0 and build
  a libgdamm-6.0 API that will be parallel-installable with
  libgdamm-5.0.
Add gen_scripts and regenerate enum and methods docs.

Add tools/gen_scripts/*.sh based on the ones now in cluttermm.
Regenerate libgda_enums.efs and libgda_methods.defs.
Use NO_GTYPE with many _WRAP_GERROR() and _WRAP_ENUMS() lines.

libgda-6.0 does not rnu glib-mkenums over all of the header
files anymore, so not all enums have *_get_type() functions.
This is apparently intentional though it was an unrelated
change in this commit:
https://git.gnome.org/browse/libgda/commit/libgda/Makefile.am?id=0111e1411cdaa9bca1169b367ef2ec26338c432b

It only prevents us from using these enums in a Glib::Value<>
so it is not a big problem.
Regenerate libgda_signals.defs.
Connection: close() can now throw an exception.

Also ignore a private function.