Scroll to Inspected Element

Summary:
changelog: Add scroll to inspected element in layout plugin

Before this diff, when one inspected an element, one needed to scroll down to see highlighted line for that element. This diff added automatic scroll to inspected element. It will scroll so that the line is in middle of the app.

Also, fix direct state mutation and this error:
```
Public property 'onKeyDown' of exported class has or is using private name 'Element'.
```

Reviewed By: passy, mweststrate

Differential Revision: D20798587

fbshipit-source-id: 763eb63cd51abd73940e301e36e89232033722c3

desktop/plugins/layout/Search.tsx

@@ -68,7 +68,7 @@ export default class Search extends Component<Props, State> {
     this.timer = setTimeout(() => this.performSearch(value), 200);
   };
 
-  onKeyDown = (e: React.KeyboardEvent) => {
+  onKeyDown = (e: React.KeyboardEvent<HTMLInputElement>) => {
     if (e.key === 'Enter') {
       this.performSearch(this.state.value);
     }