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.

Reproduza o CodePen vinculado e compare o comportamento de hover para scatterpolar, scattergl e scatterpolargl com fill definido como none e toself. Comece rastreando o tratamento de hover para esses tipos de trace; considera-se concluído quando o comportamento de hoverlabel e hoverinfo for consistente com o comportamento esperado de fill e a cobertura de regressão tiver sido adicionada onde a estrutura de testes existente permitir.