`on_hover`: pass MouseEvent to the user

[CalvinFernandez]

Currently, Plotly doesn't pass the raw MouseEvent to the user. I'd like to use clientX and clientY to determine the location of the user's cursor at the time the hover event was fired to render a custom tooltip.

would like to augment the hover event, something like on_hover: (data, event)

[rreusser]

@etpinard we've been discussing object containers for data so that we can bundle more information along with event callbacks. I don't know precisely what this means for backwards compatibility (i.e. which fraction of events do this already), but perhaps this is a use-case. I think ideally all events would pass an object (or at least a variable-list length of arguments) so that items like this could at least be added without versioning fears.

[etpinard]

Yeah, that would be nice.

I'll referenced #168 and #145 but this request deserves a ticker of its own. Thanks @CalvinFernandez

I would prefer that our plotly_ events pass one lone object as event data, so maybe the signature could be:

gd.on('plotly_hover', (eventData) => {
   eventData.event // => original Mouse Event object
});

Implementing for cartesian plots should be easy. Doing the same for our gl3d plots will be a bit of a headache though.

[rreusser]

Does this require a major version bump?

[etpinard]

Does this require a major version bump?

Not if we link the event object to the existing eventData object.

[etpinard]

cc @n-riesco

[etpinard]

Done in #1505