Improve Lumino cross-compatiblity with React

[afshin]

@jasongrout recently brought up the point that by depending on React in @phosphor/widgets, we would have to bump the major version of our library when React has a major version bump. The TypeScript definitions for React have also been historically brittle. In JupyterLab, we have been forced to declare the types as a pinned dependency.

I think instead we should make it easier to use React with Phosphor.

• Add the RenderWidget
• Utilize the fact that jsx can now be declared per module in TypeScript
• Add a pass through for React renderers to the Phosphor vdom
• Add examples of using React in a Phosphor widget and vice versa.
• Investigate allowing React components to be used as the renderers for the core Phosphor widgets.

Originally posted by @blink1073 in phosphorjs/phosphor#302 (comment)