elc_naviframe: Hide preserved content after its item is popped.

Previously, preserved content was shown unexpectedly after its naviframe
item was popped because the preserved content was not removed but was
unset from its naviframe item.

Since popped naviframe item becomes hidden on the screen, the preserved
content of the popped naviframe item should become hidden by default as
well.

src/lib/elementary/elc_naviframe.c

@@ -198,6 +198,7 @@ _item_free(Elm_Naviframe_Item_Data *it)
              elm_object_part_content_unset(VIEW(it), CONTENT_PART);
              evas_object_event_callback_del
                 (it->content, EVAS_CALLBACK_DEL, _item_content_del_cb);
+             evas_object_hide(it->content);
           }
      }
 }