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)