Adding support to plotly_unselect event

[denphi]

When active selections are cleared using double-click interaction over a plotly plot, python objects are not updated properly. This code enables plotly_deselect events to update points properly

[jonmmease]

Thanks for the PR @denphi! This will be a really helpful addition. The code looks great. I'll test it out over the next few days and then merge it in for version 3.9.

[jonmmease]

Great work @denphi! I made only one small adjustment, rather than setting the trace's selectedpoints property to () when plotly_deselect happens I set it to None. The reason is that plotly.js treats None to mean that no selection is active (and so no points are transparent) whereas () means that the selection mode is active but nothing is selected (and so all of the points are transparent).

Will merge when tests are green (all but plotlyjs_dev_build, which is known to be failing right now). Thanks again!