Broadcast channels are more suited to uBO than DOM events to dispatch
notifications to different parts of uBO.
DOM events can only be dispatched to local context, broadcast channels
dispatch to all contexts (i.e. background process, workers, auxiliary
pages) -- this last behavior is better suited to uBO to communicate
internal changes to all potential listeners, not just those in the local
context.
Additionally, broadcasting to content scripts is now done through
tabs.sendMessage() instead of through potentially opened message
ports, this simplifies broadcasting to content scripts, and this
doesn't require to have long-lived message ports in content
scripts.