Proposal: allow Child Apps to be unregistered

## Currently

We only allow registering Child Apps. But there is no public API to remove them.

## Proposal

Implement `app.unregisterApp(childAppName)` method.

The argument `childAppName` is a string, and it will look for any registered Child App with that name, and remove them. If they happen to have instances, then their instances should be properly destroyed (so that their callbacks can fire properly).

Start at the public app registration API and trace how Child Apps are registered, stored, and instantiated. Check the existing lifecycle and callback behavior for Child App instances; done means a named Child App can be removed through the proposed API and any of its instances are properly destroyed.

Written by the indexing model from the issue text.