efl_ui_position_manager: move from accessor to function callback

efl_ui_position_manager: move from accessor to function callback

this commit enables access to the item structure of the collection via a
function callback. The function callback now enables batching for items,
which does not pay off right now. However, a few more optimizations can
be done in order to get the whole payoff.

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Differential Revision: https://phab.enlightenment.org/D9445