gtk-demo: Improve flowbox demo code
gtk-demo: Improve flowbox demo code

Overriding the background color for a color swatch is wrong. The color
is not the background, it's the foreground, so it should be painted in
a draw signal handler.

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