[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.

doc/reference/clutter/clutter-sections.txt

@@ -1806,6 +1806,8 @@ ClutterBox
 ClutterBoxClass
 clutter_box_new
 clutter_box_get_layout_manager
+clutter_box_add
+clutter_box_addv
 
 <SUBSECTION Standard>
 CLUTTER_TYPE_BOX