[gtkada] Support for Glade 2 XML format

[Arnaud Charlet]

We are about to start working on supporting the Glade 2 XML files in
gate, since this is the last blocking item for making a new release of
GtkAda, which will be called GtkAda 2.2.0, and will support Gtk+ 2.2.x

So if anyone (e.g. Preben) is working on this right now, it would be nice
to send us their changes, if they are in some reasonable state, so
as to combine our efforts.

If these changes are not really ready, that's not a big deal, we will
start from scratch.

Arno