Return to app initial state in debugger

[canadaduane]

In the debugger, after I click on an event, there's no way to return to the app's "initial state" since there is no message that corresponds with that state.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[lukewestby]

Tracking in #98. Thanks!