[layout] Implement ClutterBox::add

[layout] Implement ClutterBox::add

The ClutterBox::add method is a simple wrapper around the Container
add_actor() method and the LayoutManager layout properties API. It
allows adding an actor to a Box and setting the layout properties in
one call.

If the LayoutManager used by the Box does not support layout properties
then the add() method short-circuits out.

Along with the varargs version of the method there's also a vector-based
variant, for language bindings to use.