efl_ui_position_manager: refactor data access

efl_ui_position_manager: refactor data access

before data access has been a property on
Efl.Ui.Position_Manager.Entity. However, we are not sure how stable all
this will be, and maybe we have the demand to change that in future, in
order to allow more high-performance item passing here.

Additionally, this patch addresses a few lectures learned from the past,
that includes:
- direct group header filling.
- direct group sizing
- difference between size for caching, and size for displaying
- Have a end_id which is the maximum of id that is filled.

ref T8179

Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Differential Revision: https://phab.enlightenment.org/D9754