[gnome-db] Delete GdaValue and use only GValue proporsal

["Daniel Espinosa" <esodan gmail com>]

As I know, most of the new or actual users of the libgda, may don't
know about GdaValue is now realy GValue, and that's why may mos of them
don't use the GLib's related functions about it; for this reazon I
propose to delete the GdaValue as a Type and use, for all the API and
providers developtment, GValue enstead.

-- 
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