popup: remove previous object when setting new content

Summary:
Other widgets remove previous object when setting new content, but popup does
nothing for the part whose name is "default".
content_unset can be used to keep previous content object, so content_set
(especially with NULL) should be able to delete previous content object
for uniformity of the entire widgets.

@fix

Reviewers: jpeg, singh.amitesh, cedric, raster, conr2d

Reviewed By: conr2d

Subscribers: minkyu, herb

Differential Revision: https://phab.enlightenment.org/D4885

src/lib/elementary/elc_popup.c

@@ -1128,7 +1128,9 @@ _content_set(Evas_Object *obj,
         _list_del(sd);
      }
 
+   evas_object_del(sd->content);
    sd->content = content;
+
    if (content)
      {
         if (!sd->scroll)