Add a dbp-bus-sub element for subscribing to a event bus channel
Currently subscribing to an event bus message requires you to setup the object, subscript in connected() and unsubscribe in disconnected() (otherwise the elemetn gets leaked because it stays referenced from the window object).
We could instead provide an element which can be used in combination with lit-element:
render() {
html`<dbp-bus-sub name="event-name" @callback=${this.myEventHandler}></dbp-bus-sub>`
}
This would make lit-element handle the event listener handling and correctly adds/removes the subscription when the listening element gets removed from the DOM. The downside is that it triggers the first event only after the element is rendered at least once. So if the event handler in turn triggers a state change with the subscriber then it would immediately re-render. That's probably not a big issue though.