Adds subscription and delegation support for mouseenter and mouseleave events. Unlike mouseover and mouseout, these events aren't fired from child elements of a subscribed node.

This avoids receiving three mouseover notifications from a setup like

div#container > p > a[href]

where

Y.one('#container').on('mouseover', callback)

When the mouse moves over the link, one mouseover event is fired from #container, then when the mouse moves over the p, another mouseover event is fired and bubbles to #container, causing a second notification, and finally when the mouse moves over the link, a third mouseover event is fired and bubbles to #container for a third notification.

By contrast, using mouseenter instead of mouseover, the callback would be executed only once when the mouse moves over #container.