Better selection highlighting for splom traces

Crossfiltering within a splom is perhaps its most powerful (and fun!) feature. We should make this better by:

• When there's a splom trace, make dragmode: 'select' the default, rather than 'zoom', so more people are encouraged to find and use this!
• Further fade the deselected points. splom traces encourage many small panes, which makes points overlap a lot, so the opacity we normally use for deselection builds up to the point you can't distinguish selected from deselected very well. We should decrease the opacity, and/or partially desaturate the colors, by default. Here's an example from https://plot.ly/python/splom/ - off-diagonal you can certainly see the selection, but it doesn't really pop out as much as it could. On diagonal, where the overlap is much greater, you can't even really see the selection:

ファイル、テスト、エントリポイントは指定されていません。splom のトレース処理を見つけ、次に dragmode のデフォルト値と選択解除されたポイントのスタイルがどのように選択されるかを調べます。両方のデフォルト値を変更する前に、既存の選択動作と比較してください。splom が select mode で開始し、密集したペインでも選択されたポイントが明確に区別できれば完了です。