Set the way child is packed in self

Property definitions

gtk $ GtkBox :: set_child_packing
	# Set the way `child` is packed in `self`
	fun set_child_packing(child: GtkWidget, expand, fill: Bool, padding: Int, packing: GtkPackType) `{
		gtk_box_set_child_packing(self, child, expand, fill, padding, packing);
	`}
lib/gtk/v3_4/gtk_core.nit:439,2--442,3