selectedpoints in FigureWidget not updated on selection

[jonmmease]

See https://community.plot.ly/t/lasso-deselect-from-inside-a-function/19112/2 for background.

It seems like the scatter.selectedpoints property does not get updated automatically on selection events. Because of this, setting it to [] to clear a selection only works the first time, because after the first time the Python side thinks the value is already []

This might simply be a matter of needing to sync the properties on plotly_selected events.

[jonmmease]

Done in 675633e