box: Convert to gtk_container_child_notify_by_pspec
box: Convert to gtk_container_child_notify_by_pspec

For the same reasons as g_object_child_notify_by_pspec.

grubersjoe/adwaita,jadahl/gtk,grubersjoe/adwaita,davidgumberg/gtk,davidgumberg/gtk,jadahl/gtk,alexlarsson/gtk,jigpu/gtk,grubersjoe/adwaita,davidgumberg/gtk,jadahl/gtk,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,jigpu/gtk,grubersjoe/adwaita,jadahl/gtk,alexlarsson/gtk,jadahl/gtk,jigpu/gtk,davidgumberg/gtk,jigpu/gtk,jadahl/gtk,alexlarsson/gtk,jadahl/gtk,grubersjoe/adwaita,davidgumberg/gtk,jadahl/gtk,davidgumberg/gtk,davidgumberg/gtk,grubersjoe/adwaita,jigpu/gtk,alexlarsson/gtk,alexlarsson/gtk,davidgumberg/gtk,jigpu/gtk,jigpu/gtk,alexlarsson/gtk,grubersjoe/adwaita