Provide a widget map feature to help users locate code from UI

## Problem

When working on an unfamiliar codebase, the user can have difficulty finding the widget code they are asked to modify in a given screen.

## Proposal

Add a widget map feature which shows preview images for all the screens in a Flutter project and allow the user to inspect widgets in each screen to locate the code for a specific widget.

In addition, since user study shows that people tend to start examining a Flutter codebase from `pubspec.yaml` and `main.dart`. We could show a mini widget map when the user opens these two files to improve discoverability.

Here is a mockup for this concept: