More robust event mechanism

[ccbrown]

In particular, components should be able to dispatch events, and subscribers should be able to choose to either consume them or give another subscriber a chance to handle them.

This could look something like...

hooks.use_events<MyEvent>(|e| {
    if ... {
        // handle the event
        e.stop_propagation();
    }
});

Somewhere else...

let dispatcher = hooks.use_event_dispatcher<MyEvent>();

dispatcher.dispatch(MyEvent::new(...));

Unless stop_propagate is called on the event, the event would bubble up the chain of parent elements with event handlers for that type.

This mechanism could potentially replace the current use_terminal_events hook, if for keystroke events we also had a mechanism to define a focus or "first responder" to which they would be dispatched initially.