fix: Unknown event handler property onEvent (#2)

* fix: Unknown event handler property `onEvent`

```
index.js:1 Warning: Unknown event handler property `onEvent`. It will be ignored.
	in div (created by Content)
    in Content (created by Editor)
```

* fix: make onEvent required

Seems this prop is provided in `component/editor.js`

Author: Timofey Dergachev

packages/slate-react/src/components/content.js

@@ -57,6 +57,7 @@ class Content extends React.Component {
     contentKey: Types.number,
     editor: Types.object.isRequired,
     id: Types.string,
+    onEvent: Types.func.isRequired,
     readOnly: Types.bool.isRequired,
     role: Types.string,
     spellCheck: Types.bool.isRequired,