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.

Reproduisez le CodePen lié et comparez le comportement du survol pour scatterpolar, scattergl et scatterpolargl avec fill défini sur none et toself. Commencez par suivre la gestion du survol pour ces types de trace ; le travail est considéré comme terminé lorsque le comportement de hoverlabel et hoverinfo est cohérent avec le comportement attendu de fill et qu’une couverture de régression a été ajoutée là où la structure de tests existante le permet.