[gnome-db] GdaValue::is_null and gda_data_model_is_editable()

[Paisa Seeluangsawat <paisa unt edu>]

Forgot to ask whether people want GdaValue::is_null to be implemented
as a separate bool, or a bit set on type.  I'm inclined toward the
latter to save space.

I'll also split gda_data_model_is_editable() into _is_updatable() and
_is_insertable().  The reason is in,

  http://mail.gnome.org/archives/gnome-db-list/2003-October/msg00150.html

If there's any objection, please speak now or forever hold your peace.