Add maintainSnapping method to OutputModel

The intent here is to impact the position of screens around a given
screen when its size changes. This way we avoid weird overlaps or gaps
on resize with the screens snapped to a given one.

Note that even though it'll fully respect snapping in one direction,
it won't respect snapping between other screens in different directions.
This would require a full fledged layout engine which would be
complicated to grow in the current model.

Since it's also a less common case, I chose to ignore it. Let's make it
work for the 80% of the cases which are generally in one direction only.