Add support for React-like custom element functions

[gnestor]

This allows phosphor to render custom elements in JSX defined as:

const CustomElement = props => <button onClick={props.onClick}>{props.label}</button>

[sccolbert]

Thanks for this!

Because of the camel-case changes, this is a breaking API change. If I release a 2.0 of this, it will force widgets to go to 2.0 as well. I'd rather not do that right now, so I'd like to thing of a way we can support both. I have an idea of how to this, just buried under grid work right now.

[gnestor]

Ok, let's move those commits into a separate PR. I think the element event/attribute names will require some more work to get 100% parity with React.

[sccolbert]

I wonder if we should put the JSX definitions into a separate .d.ts file. If they are made global like this, I don't think anyone will be able to use the react typings when using phosphor.

[gnestor]

I am new to Typescript so that sounds like a good plan 👍

[sccolbert]

Possibly related: microsoft/TypeScript#8757

I wonder if we can figure out how to define the namespace in Phosphor, then import and use it in another module without needing to make it global for the whole app.

[ellisonbg]

As far as the JSX namespace thing. As long as it is global I worry folks can't use react at the same time. Does putting it into a separate d.ts file help that? The problem is that the typescript compiler is currently expecting JSX to be global, so putting it as a module level namespace doesn't work.

[sccolbert]

@ellisonbg The typescript issue I linked seemed to be asking for that, and sounds like from the responses it's possible. I may be misunderstanding the conversation though.

[jdetle]

How is this looking wrt possibly merging it?

[ellisonbg]

I don't think we should merge this PR as we are wanting to maintain less VDOM stuff in phosphor, not more. Also, the JSX typings do conflict with @types/react. While we haven't made a final decision, it is my preference for us to simply user react and deprecate phosphors vdom.