[gtkada] Support for Glade 2 XML format

[Nicolas Setton]

Work has now started on integrating support for the Glade 2 XML format.
Preben Randhol's contributions have been integrated in the development
branch of the CVS tree. Many thanks, Preben :-)

There are still 69 widgets for which the new format need to be
supported. They are marked with the comment "--  ??? Need to re-sync" in
the file gtk_generates.adb. Those of you who would like to contribute by
converting those functions, don't hesitate to contact us ;-)

Nico, for the GtkAda team.