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:

Non sono indicati file, test o punti di ingresso. Individua la gestione delle tracce splom, quindi esamina come vengono selezionati i valori predefiniti di dragmode e lo stile dei punti non selezionati; confronta il comportamento di selezione esistente prima di modificare entrambi i valori predefiniti. Il lavoro è completo quando splom si avvia in select mode e i punti selezionati rimangono chiaramente distinguibili nei riquadri densi.