feat(Dropdown): Pass object with name to onChange

[jeffcarbs]

Fixes #218

This is a breaking change but I think it's worth it. There's currently no easy way to get the name of the Dropdown in the onChange event. Normally in an onChange the event.target will be the input that received the change. So in a form, for example, you can attach the same change handler to multiple elements and use the event to determine the appropriate field/value to update.

For Dropdown that's not the case - it will likely be a DropdownItem.

The Checkbox is an example of a component that faces a similar issue regarding the target. In that component, the onClick and onChange are both called with { name, value, checked }. This PR updates Dropdown to be called with { name, value }.

I also cleaned up the naming of our internal event handler to be handleChange. The pattern of this project seems to be:

• handle<event> for our internal handlers
• on<event> is a prop that the user passes in and is called by our internal handlers if set.

[jeffcarbs]

This also uncomments the test referenced in https://github.com/TechnologyAdvice/stardust/issues/218. I grepped for all of our event handlers and it seems that they are all called with the event first so I think that can be closed with this PR.

[jeffcarbs]

Actually realized that onAddItem wasn't receiving an event first. This updates the arguments so the event is passed first then { name, value }.

So two breaking changes in this PR, both in Dropdown:

-onChange(e, value)
+onChange(e, { name, value })

-onAddItem(value)
+onAddItem(e, { name, value })

[codecov-io]

Current coverage is 99.62% (diff: 100%)

Merging #581 into master will decrease coverage by <.01%

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[levithomason]

Cool, looks good. I think we should always pass (e, data) where data is an object. Just to keep it consistent and extensible.

I'll try to review/merge/ship this tonight.