When all the items in the legend don't fit in the designated space, a navigation component will be added to the legend, which will make it possible to navigate to items currently not in view.
The navigation contains two <button> elements whose content can be modified through the navigation setting.
Scroll to the next item.
Scroll to the previous item.
Trigger a manual scroll by passing the amount of pixels as a parameter.
This event is to allow mousewheel scroll and scroll on panning.