efl_ui/box: optimize position_set operations with boxes

efl_ui/box: optimize position_set operations with boxes

if a box is moved and no other changes are made to the box or its children,
e.g., if the box is scrolled, then there is no need to loop over the box's
items repeatedly in order to accurately calculate all the item geometries
and positions.

instead, simply apply an offset from the last box calc position to each child
item and handle the position changes more transparently

this yields roughly a 12% perf improvement to the 'efl.ui.scroller simple' test
and brings rendering up to nearly 60fps

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Differential Revision: https://phab.enlightenment.org/D9342