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.