Implement `hoveron: 'fills'` for scattergl and scatterpolargl

During a discussion in the plotly.py forums in came up that the hoverlabel for the scatterpolar trace has a different behavior when the fill state is set to toself than when it's set to none.

CodePen: https://codepen.io/anon/pen/EpOGdX

fill: 'none' for scatterpolar

fill: 'toself' for scatterpolar

It also seems that the hoverinfo propery has no effect when fill is toself.

I did the same comparison with the scatterpolargl trace and it has the behavior that I expected:

fill: 'toself; for scatterpolargl

I don't know if this is a bug in scatterpolar or scatterpolargl, but I find the current scatterpolargl behavior (The behavior consistent with fill of none) to be more helpful.

Reproduce el CodePen enlazado y compara el comportamiento del hover para scatterpolar, scattergl y scatterpolargl con fill establecido en none y toself. Empieza siguiendo el manejo del hover para estos tipos de trace; se considera terminado cuando el comportamiento de hoverlabel y hoverinfo sea coherente con el comportamiento esperado de fill y se haya añadido cobertura de regresión donde la estructura de pruebas existente lo permita.