Sheet component for plugins

Summary:
Exposing a `<Sheet>` component to plugins. This allows plugins to create sheets, by returning something like this from their render function:

```js
return <div>
  contents of my plugin
  <Sheet>{() => <div>content of my sheet</div>}
</div>
```

Reviewed By: passy

Differential Revision: D13597251

fbshipit-source-id: 9da6ba6d2036243ddd2d05b73d392bf24be8d375

src/ui/index.js

@@ -165,3 +165,5 @@ export {
 export {InspectorSidebar} from './components/elements-inspector/sidebar.js';
 
 export {Console} from './components/console.js';
+
+export {default as Sheet} from './components/Sheet.js';