Add persistent layout support

[clemencov]

Persistent layout implementation, inspired by closed PR #2440.

[frol]

How exactly does this help given there is a natural way of compositing React components: https://github.com/zeit/next.js/tree/canary/examples/layout-component ?

[clemencov]

State persistence over page transitions (media players, notifications, chat scroll positions).
Few discussions in: #3200, #88

[frol]

@clemencov

This actually won't fix #88 since ReactDOM.render is still called on page changes.

(c) @timneutkens (#2440 (comment))

[clemencov]

Yes, it doesn't help much with animations, but until renderReactElement function is not calling ReactDOM's unmountComponentAtNode, React will not unmount the Layout component and relying to this we can use it as a persistent top-level component.

[rauchg]

I definitely want this, but I don't think we need the concept of a single global layout. One of the ideas I had was that we could put a key or attribute on the top level component. If it matches the one of the next page, we retain that top level component.

[tz5514]

@rauchg Is there any schedule for this important feature?
Lots of developer want this #88.
We really really hope that this feature can be implemented as soon.
Thanks!

[timneutkens]

Closing in favor of #4129