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.

复现链接的 CodePen，并比较将 fill 设置为 none 和 toself 时 scatterpolar、scattergl 和 scatterpolargl 的 hover 行为。首先跟踪这些 trace 类型的 hover 处理；当 hoverlabel 和 hoverinfo 的行为与预期的 fill 行为一致，并且在现有测试结构允许的情况下添加了回归覆盖时，即表示完成。