elm_scroller: update behaviour to match legacy behaviour more closely

elm_scroller: update behaviour to match legacy behaviour more closely

This implements scrolling the scroller when the scroller is not in the
maximum position of the requested relation, but there is no more focus
object in that relation.

Summing up, the scroller is scrolled manually, when the focus object is
not fitting in the viewport, and the object is beeing out of the
viewport in the requested direction. Or the scroller is not at its max
position in the requested direction, and there is no more focusable
object in this requested direction.

ref T6804

Differential Revision: https://phab.enlightenment.org/D7382