Add visualiser controls + target mode

Summary:
Now that we have panels for tree visualiser separately we can have visualiser specific controls. There is a dedicated button for focus mode which should make that more discoverable and a better implementation of target mode which uses a slider. This has several benefits:
1. more discoverable
2. more obvious what is going on with the text prompts and a real slider control instead of mouse enter
3. there is no context menu getting in the way of the content

Changelog: UIDebugger Add visualizer target mode feature for selecting views in the z stack easily
Changelog: UIDebugger Add FocusMode button to visualiser toolbar

Reviewed By: mweststrate

Differential Revision: D47671658

fbshipit-source-id: 6f657f9d417280627457624660b934c9898cda58

desktop/plugins/public/ui-debugger/plugin/uiActions.tsx

@@ -90,7 +90,7 @@ export function uiActions(
         tracker.track('node-focused', {name: focusedNode.name, tags});
       }
 
-      uiState.selectedNode.set(undefined);
+      uiState.selectedNode.set({id: node, source: 'visualiser'});
     }
 
     uiState.focusedNode.set(node);