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:

No se nombran archivos, pruebas ni puntos de entrada. Localiza el manejo de trazas splom y, a continuación, inspecciona cómo se seleccionan los valores predeterminados de dragmode y el estilo de los puntos deseleccionados; compara el comportamiento de selección existente antes de cambiar ambos valores predeterminados. La tarea estará terminada cuando splom se inicie en select mode y los puntos seleccionados sigan siendo claramente distinguibles en paneles densos.