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.

リンクされたCodePenを再現し、fillをnoneおよびtoselfに設定した場合のscatterpolar、scattergl、scatterpolarglのホバー動作を比較します。まず、これらのtraceタイプのホバー処理を追跡します。hoverlabelとhoverinfoの動作が想定されるfillの動作と一貫しており、既存のテスト構成で可能な箇所にリグレッションテストのカバレッジが追加されていれば完了です。