Support listening to ChannelEvent objects in endproject webapps

At [1] and sibling pages, it is described how end projects can subscribe to certain events. However, we need to reconsider how this should be done for version 13.0.0 : In version 13.0.0, the hst-pagecomposer will 'live' in the platform webapp. When a channel gets, say published, a publication channel event is posted to the synchronous HST eventbus, however this is the HST eventbus in the platform webapp. Endprojects at this moment won't receive these events if you follow [1], because the listeners are subscribed to the Spring component manager of the endproject webapp, which does not get the event.

We need to think about how we'd like to support this in version 13.0.0

[1] https://www.onehippo.org/library/concepts/channels/channel-manager-events.html