Add child and pack it at the start of the box

Property definitions

gtk $ GtkBox :: pack_start
	# Add `child` and pack it at the start of the box
	fun pack_start(child: GtkWidget, expand, fill: Bool, padding: Int) `{
		gtk_box_pack_start(self, child, expand, fill, padding);
	`}
lib/gtk/v3_4/gtk_core.nit:429,2--432,3