Overlay Notifications

I like the idea of just keeping the default messages app/widget with the library and deferring to custom apps if installed.

Maybe we could even do something like this:

// in message library:
Bangle.emit("message", event);
setTimeout(()=>{if (!event.handled) load("messages.app.js");});

// some music widget
Bangle.on("message", event=> {
        if (event.handled || event.id != 'music') return;
        event.handled = true;
        /* display music info */
});

// custom messages boot.js
Bangle.on("message", event => {
        if (event.handled) return;
        event.handled = true; // probably not even needed?
        load('custommessages.app.js');
});