GtkDialog: Be more careful in handling the headerbar
GtkDialog: Be more careful in handling the headerbar

Just because an action widget has a parent thats a headerbar,
it is not guaranteed that it is 'our' headerbar. Add some
assertions to enforce this.

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