Add support for highlighting a source on hover on its legend item

This introduces a new property, highlight on charts that, when set, will display the specified item "highlighted", which effectively means reducing the visibility of the other items. It also adds hover highlighting to Legend, which will highlight a legend entry on hover and exposes the highlighted index. Combined this allows charts to highlight an item when its corresponding legend entry is hovered, which helps clarify which item corresponds to which legend entry. There's also a bunch of refactoring changes included that clean up the charts example.

Screenshots:

	Unhighlighted	Highlighted
Bar Chart
Pie Chart
Line Chart