efl_ui_position_manager_list: make it handle group items
this makes the group items stick at the top of the viewport, if the
corresponding items in there do have the item available.
For now items between two groups are not really handled, the group
header will still just be displayed.
The code for this feature is explicitly written in a single block, it
was said that we might want to have this able to be enabled / disabled
This commit also shuffels the code here a bit, one single method just
got too long.
Reviewed-by: Cedric BAIL <email@example.com>
Differential Revision: https://phab.enlightenment.org/D9587