edje: implement container api

edje: implement container api

the container api returns the number of added swallows.
the content iterator returns a iterator over all swallowed objects.

The reason this is only for swallows is that the interface explicitly
mentions UI elements at the count. Since i am believing the the result
of the count operation should mirror to the amount of elements in the
iterator, the iterator only returns swallowed objects.

ref T5719

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Differential Revision: https://phab.enlightenment.org/D7748