listbox: Scroll headers on screen
listbox: Scroll headers on screen

The listbox code relies on the container focus adjustment handling
to scroll the cursor row on screen. But GtkContainer has no idea
about row headers, so ensure that we scroll the header on screen too.

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