gtk-demo: Fix expand button box demo
gtk-demo: Fix expand button box demo

Set the spacing to 0, so the buttons are actually linked.

Lyude/gtk-,chergert/gtk,Lyude/gtk-,ahodesuka/gtk,chergert/gtk,Lyude/gtk-,jessevdk/gtk,alexlarsson/gtk,Lyude/gtk-,jessevdk/gtk,davidgumberg/gtk,davidgumberg/gtk,Lyude/gtk-,Adamovskiy/gtk,alexlarsson/gtk,jadahl/gtk,jigpu/gtk,davidgumberg/gtk,grubersjoe/adwaita,ahodesuka/gtk,jadahl/gtk,davidgumberg/gtk,Adamovskiy/gtk,jigpu/gtk,Adamovskiy/gtk,Adamovskiy/gtk,grubersjoe/adwaita,chergert/gtk,chergert/gtk,ahodesuka/gtk,jigpu/gtk,chergert/gtk,davidgumberg/gtk,ahodesuka/gtk,jigpu/gtk,Lyude/gtk-,Adamovskiy/gtk,chergert/gtk,chergert/gtk,ahodesuka/gtk,alexlarsson/gtk,jadahl/gtk,Lyude/gtk-,chergert/gtk,grubersjoe/adwaita,ahodesuka/gtk,ahodesuka/gtk,davidgumberg/gtk,jessevdk/gtk,Adamovskiy/gtk,jigpu/gtk,jadahl/gtk,grubersjoe/adwaita,davidgumberg/gtk,alexlarsson/gtk,ahodesuka/gtk,grubersjoe/adwaita,jigpu/gtk,jessevdk/gtk,jessevdk/gtk,jadahl/gtk,jadahl/gtk,Adamovskiy/gtk,alexlarsson/gtk,alexlarsson/gtk,jadahl/gtk,alexlarsson/gtk,jigpu/gtk,jadahl/gtk,jessevdk/gtk,jigpu/gtk,davidgumberg/gtk,grubersjoe/adwaita,Adamovskiy/gtk,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,Lyude/gtk-,jessevdk/gtk