slate-react: MVP for working with non-global window objects (fix for #3819) (#4079)

* mvp implementation for working with non-global window instances

* remove unused element renderer

* fix typo in comment

* fix wrong example reference

* Add @babel/helper-call-delegate to fix build error

Co-authored-by: Lukas Buenger <lukasbuenger@gmail.com>

site/pages/examples/[example].tsx

@@ -25,6 +25,7 @@ import RichText from '../../examples/richtext'
 import SearchHighlighting from '../../examples/search-highlighting'
 import CodeHighlighting from '../../examples/code-highlighting'
 import Tables from '../../examples/tables'
+import IFrames from '../../examples/iframe'
 
 // node
 import { getAllExamples } from '../api'
@@ -48,6 +49,7 @@ const EXAMPLES = [
   ['Search Highlighting', SearchHighlighting, 'search-highlighting'],
   ['Code Highlighting', CodeHighlighting, 'code-highlighting'],
   ['Tables', Tables, 'tables'],
+  ['Rendering in iframes', IFrames, 'iframe'],
 ]
 
 const Header = props => (