[gtkada] base_widget example fails Assert

[Arnaud Charlet]

Thanks for the patch.

Most of your changes have been applied to CVS, with additional changes.
In particular, you've reformatted continuation lines with 3 characters instead
of 2.

> I used 'pragma Unreferenced', which is only in gnat 3.15; not sure if
> that's appropriate either.

That'a appropriate for the HEAD.

> Let me know if you'd rather have this in some other format, or smaller
> pieces.

This format was fine.

Arno