Create a new GtkBox with the given orientation and spacing between its children

Property definitions

gtk $ GtkBox :: new
	# Create a new `GtkBox` with the given `orientation` and `spacing` between its children
	new (orientation: GtkOrientation, spacing: Int) `{
		return (GtkBox *)gtk_box_new(orientation, spacing);
	`}
lib/gtk/v3_4/gtk_core.nit:416,2--419,3