Co-inspect: link from code to the running program

The Inspect dialog, implemented by @three, already links shapes in a running CodeWorld program to the corresponding code (via the inspect dialog). We also want the opposite. When a program is paused or in "inspect mode", we could collect the set of all source locations that map to objects in the picture, and offer a UI to get from code to the right place in the picture tree, as well.

Idea comes from an old article by Bret Victor (http://worrydream.com/#!/LearnableProgramming)

Start with the Inspect dialog implemented by @three and trace how it links shapes in a running CodeWorld program to source locations. Define and implement the paused or inspect-mode interaction that lets a source location navigate to the corresponding object in the picture tree, then verify both directions work.

Written by the indexing model from the issue text.