widget: Move item_loop_enabled to scrollable (EO)

widget: Move item_loop_enabled to scrollable (EO)

I was told that the scrollable interface is being redesigned for EO.
This API definitely does not belong to the base Widget class, as it's
quite specific to item-based scrollable widgets, such as lists and
grids. Since Elm.Interface_Scrollable is itself being revamped, it is a
good place to move that EO API for now.

Ref T5363