Explainer: Separate APIs for app-scoped badge vs document badge (#51)

Update explainer to new "split API" design.

Closes #49 (This is the main issue being addressed).
Closes #55 (The proposal that this change implements).

Also, incidentally:
Closes #43.
Closes #52.
Closes #53.

- Splits the API into separate "app" and "client" methods.
- Drops the concept of "handles", instead just explicitly focus on apps.
- Moves API from Badge namespace to navigator.
- Removes the explicit "scope" option (setAppBadge just applies to most specific app scope).