[gtkada] Remove space between buttons

[Emmanuel Briot]

> I'm not sure if it's about the Gtk_Table, because when I put my mouse on
> a button it highlight itself with a small purple border. I've updated
> the screenshot so you can see what I mean:
> http://www.borer.name/files/eivd/screen_button.png

Nice experiment.
Have you looked at the drawing at the bottom of gtk-button.ads ? It 
describes all the margins that apply to a button. In particular,
    Set_Border_Width (Button, 0);
might help if not done already