Add legend click interaction attrs

[etpinard]

currently named itemclick and itemdoubleclick, both enumerated attributes with values 'toggle',
'focus' (now 'toggleothers'), false. This PR could be viewed as a follow-up of #2581

resolves #665 by setting

layout = {
  legend: {
    itemclick: false,
    itemdoubleclick: false
  }
}

resolves #674 by setting

layout = {
  legend: {
    itemclick: 'focus'
  }
}

and should help us offer solutions for #1462 and #3135

---

cc @plotly/plotly_js

[antoinerg]

Codepen with the new feature: https://codepen.io/antoinerg/pen/XwpvaO

[antoinerg]

Once a legend item is focused and we want to focus on another one, we have to first unfocus (it requires 2 actions):

It might be nice to cut that down to 1 action: it could focus right away on the newly selected legend item. In order to unfocus and display all traces, one would have to click on the legend item currently having the focus.

Now I understand this would be a breaking change and I am not even suggesting that we include this new mode in this PR. However, we should probably think ahead of time what its name would be. Maybe it could be called focus and the current focus could be called toggleothers as suggested by @nicolaskruchten?

[antoinerg]

Thank you for adding customizability to our legend interactions @etpinard

💃 💃