Introduce more general notification capabilities

[karlseguin]

Replaces the existing, very specialized Notification with something more general.

Currently, the existing page_navigate and page_navigated have been migrated.

Telemetry's page navigation event now also hooks into these events to generate the telemetry record.

There's a comment on the Notification class that talks about scoping, but...

I initially thought I'd have a global singleton, and it would be really easy to register and dispatch events. But then I realized that wouldn't work. If everything's a global, and CDP is registered for "http_bytes_received", then when Telemetry makes an HTTP request, and the client emits a "http_bytes_received" message, CDP will get those messages.

I realized things needed to be scoped. I thought about introducing a "scope_id", but decided to create a notification-per-scope. Currently, there's a Notification per Browser and one Notification for the App. I don't expect more. You can form notification topologies, which is exactly how Telemetry now works. Telemetry listens to the notification_created event from app.notification, and then registers for the page_navigate on the newly created notification instance.

[karlseguin]

I'd rather merge this after the isolated world changes are settled.