efl_ui_item: rework parent relation
this commit ensures that there is a public way of setting the container
to a item. This was also like this before, just with the difference that
it was set and unset in all privacy, however, we want to have this class
also ready for bindings, so we should stop making things privat like
Note: the removed fields from grid and list have not been used.
Reviewed-by: SangHyeon Jade Lee <firstname.lastname@example.org>
Differential Revision: https://phab.enlightenment.org/D9262