feat(legend): Pass visibility state to legend item's event callback

[netil]

Issue

#3897

Details

Pass visibility state argument to event callbacks

legend
     item: {
         onclick: function(id, visible) {
          	// toggle based on the data visibility
          	this[visible ? "hide" : "show"](id);
         },
         onover: function(id, visible) { ... },
         onout: function(id, visible) { ... },
    }
}

[coveralls]

coverage: 94.247% (+0.02%) from 94.227%
when pulling a093fb6 on netil:legend#3897
into 6f69e97 on naver:master.