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?

src/lib/edje/edje_legacy.c

@@ -122,8 +122,7 @@ edje_object_part_external_param_type_get(const Eo *obj, const char *part, const
 EAPI Evas_Object *
 edje_object_part_external_object_get(const Edje_Object *obj, const char *part)
 {
-   Edje *ed = _edje_fetch(obj);
-   return _edje_object_part_external_object_get(ed, part);
+   return efl_content_get(efl_part(obj, part));
 }
 
 /* Legacy only. Shall we deprecate this API? */

src/lib/edje/edje_part_external.c

@@ -132,4 +132,11 @@ _efl_canvas_layout_external_external_param_type_get(Eo *obj, void *_pd EINA_UNUS
    RETURN_VAL(_edje_object_part_external_param_type_get(pd->ed, pd->part, name));
 }
 
+EOLIAN static Efl_Gfx *
+_efl_canvas_layout_external_efl_container_content_get(Eo *obj, void *_pd EINA_UNUSED)
+{
+   PROXY_DATA_GET(obj, pd);
+   RETURN_VAL(_edje_object_part_external_object_get(pd->ed, pd->part));
+}
+
 #include "efl_canvas_layout_external.eo.c"

src/lib/edje/efl_canvas_layout_external.eo

@@ -1,6 +1,7 @@
 import edje_types;
 
-class Efl.Canvas.Layout.External (Efl.Canvas.Layout_Internal, Efl.Canvas.Object)
+class Efl.Canvas.Layout.External (Efl.Canvas.Layout_Internal, Efl.Container,
+                                  Efl.Canvas.Object)
 {
    [[Class representing an external part in Edje layouts.
 
@@ -14,6 +15,9 @@ class Efl.Canvas.Layout.External (Efl.Canvas.Layout_Internal, Efl.Canvas.Object)
      Or in pseudo-C:
        efl_text_set(efl_part(layout, "title"), "hello");
 
+     The real widget represented by this part can be retrieved by calling
+     @Efl.Container.content.get.
+
      @since 1.20
    ]]
    data: null;
@@ -85,4 +89,7 @@ class Efl.Canvas.Layout.External (Efl.Canvas.Layout_Internal, Efl.Canvas.Object)
       }
       */
    }
+   implements {
+      Efl.Container.content { get; }
+   }
 }