notifications: Allow applications to set the feedback profile

It's sometimes useful to emit less feedback when a notification triggers an event (or none at all). This allows for that. It's the first bit of simplifying the event-feedback <-> notfication interaction.

See the individual commits for details. Since we'll have more in that area add some documtation bits we can build on in the future too.

Closes: #730 (closed) (we use a different hint but passing none achieves the same effect)