efl_ui: rename list empty item to list placeholder item.
legacy full style item is introduced Efl.Ui.ListEmptyItem Class in new Efl Interface,
but using "Empty" name is too ambiguous to present style usage.
Thanks to @cedric and @segfaultxavi,
I found better name for this class, Efl.Ui.ListPlaceHolderItem,
as item hold the place which need to be replaced and relayouted by user generated content.
Depends on D8582
Reviewers: cedric, segfaultxavi, eagleeye
Reviewed By: eagleeye
Differential Revision: https://phab.enlightenment.org/D9034