edje: Also implement content_get on external part

edje: Also implement content_get on external part

This implements edje_object_part_external_object_get() using
efl_content_get() on the part object. Note that there are now
two ways to call APIs on the external part:
 - direct call to the efl_part() as if it was the object itself
     (implemented by composition),
 - get a handle with efl_content_get(efl_part()) and manipulate
      it directly (it is the real object).

Do we need this? Do we need the composition trick? Should we have
only one of those solutions implemented?