The default behavior of dragging and wheel zooming feels specific to 2D maps. So I think it makes sense to introduce a special type of controls (ol.control.DefaultControl) that implement default behaviors. This change also re-introduces the Navigatin control, which is a container for the default behaviors that were previously defined in the map handlers. Maybe this control needs to be renamed to Navigatin2D in the future, and there could be a different Navigation control for 3D maps.
If MapHandler subclasses could live without a map, it would make sense to have a constructor with a target and an element. But because the target is always the map, and the handler need to know about the map for performing default behavior, we always assume the map's viewport as element.
