Setting options for children added to NbtkBoxLayout is not convenient
since we are missing the varargs methods of clutter_container.

Patch in:

 child_set() - set properties of a child
 add() - add a child and set properties (this is different from
         clutter_container_add()! I think the deviation is
         with avoiding the awkward name add_with_properties()
         which is what might be expected. ClutterContainer
         currently doesn't have a method like this at all.)

The code is written to allow patching into multiple ClutterContainer
classes but for now only NbtkBoxLayout is patched, since it's the only
container we are using where we need to set options as properties.