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:

Aucun fichier, test ou point d’entrée n’est nommé. Repérez la gestion des traces splom, puis examinez comment sont sélectionnées les valeurs par défaut de dragmode et la mise en forme des points désélectionnés ; comparez le comportement de sélection existant avant de modifier ces deux valeurs par défaut. La tâche sera terminée lorsque splom démarrera en select mode et que les points sélectionnés resteront clairement distinguables dans les panneaux denses.