Persistent highlighting should only addTraces() for *new* selections

[cpsievert]

For instance, this example shouldn't keep stacking every time a selection is made:

library(plotly)

mtcars %>%
  dplyr::count(vs) %>%
  crosstalk::SharedData$new(~vs) %>%
  plot_ly(x = ~factor(vs), y = ~n) %>%
  add_bars() %>%
  layout(barmode = "stack") %>%
  highlight("plotly_click", persistent = TRUE)

This will also help avoid the issue reported in plotly/plotly.js#1467