Events that don't emit any data always get the event priority treatment

[cpsievert]

For example, run

https://github.com/ropensci/plotly/blob/master/inst/examples/shiny/event_data_persist/app.R

(1) Hover on the points to paint them red
(2) Double-click returns points to black (as expected)
(3) Hover on points again
(4) Double-click doesn't return points to black (not expected)

This is due to the fact that "plotly_doubleclick" doesn't emit any data and is currently getting treated like an input value (without event priority).