API: Add 'ref' for primitives for direct access to Node

[bryphe]

This change proposes adding a ref to the primitive nodes, that gives access to the underlying node. This isn't meant to be a common scenario by any means, but there are a few reasons I think we need this:

• For a control like <Slider />, we need to know the calculated layout values (the width/height) of the component. We can get that directly via the node, but perhaps a better event would be useful for that down the road - like onLayoutChanged?
• For a widget like <Clickable />, we really want to capture the mouse input at the start of the mousedown gesture, and then, when it is released, check if the mouseup occurred in the bounds of the widget. We can use the hitTest method on the node for this.
• For focus management - we need some underlying handle to call focus on - I think the node is a natural choice

I'm open to other ideas for this (exposing ref always feels dirty 😄 ), but this can at least help us move forward on some of these other UI widgets.

[OhadRau]

Everything here looks pretty good to me. The one question I have is whether we want to use the term id here, as this could cause some confusion to people coming from HTML/if we want to use external stylesheets in the future. Apart from that, this is good to go in my opinion.

[bryphe]

Thanks for the review, @OhadRau ! 🎉

The one question I have is whether we want to use the term id here, as this could cause some confusion to people coming from HTML/if we want to use external stylesheets in the future.

Good point, I didn't think about this - I could see confusion. Do you have any alternatives? The only thing I can think of is perhaps making it uniqueId, but I'm not sure if that is better?

[OhadRau]

Maybe something like internalId or nodeId? Can't really think of anything that makes it clear what this is either.

[bryphe]

Thanks for the suggestions, @OhadRau ! Went with internalId 👍