Add events to polar plots

[waegen]

This PR adds following events to Polar Plots in plot_api. See plotly#1463

• plotly_hover
• plotly_unhover
• plotly_click

The arguments contain followin data.
{ event: d3.event points: [], traceIdx: number, valueIdx: number }

[etpinard]

Thanks for giving this a shot!

In all honesty, although this is a major improvement over the current state of affair, it would be nice to make hover for polar charts behave a little more like on cartesian charts.

For example, hover is triggered not just when the cursor is on physically top of a marker points. Instead, we compute the distance between the cursor and marker points to determine which point is closest. Ideally, we should try to reuse Fx.hover, but formatting polar data to accordingly.

Moreover, to be more aligned with other event data object, traceIdx should be curveNumber and valueIdx should be pointNumber.