Legend event method customization does not provide consistent behavior

When customizing the methods for a legend, providing a new `onclick` method completely overrides the default click behavior
e.g.

```
if (config.legend_item_onclick) {
config.legend_item_onclick.call($$, id);
} else {
//...
}
```

However, the corresponding `mouseover` and `mouseout` events do not conform to this same behavior, they instead perform their default action AND the custom action.
e.g.

```
$$.d3.select(this).classed(CLASS.legendItemFocused, true);
if (!$$.transiting && $$.isTargetToShow(id)) {
$$.api.focus(id);
}
if (config.legend_item_onmouseover) {
config.legend_item_onmouseover.call($$, id);
}
```

This makes it difficult to truly provide a custom implementation of the legend since you are always getting some of the default behavior. Specifically this makes customizing the behavior on a touch-screen device difficult because you do not get the same `mouseover`->`mouseout` transition you get with mouse-based devices.