Deprecate event listener definitions not specifying explicit events

An event listener definition that does not specify explicitly a list of events will need to receive and filter manually in code all the platform events. This is inefficient and error-prone, and usually the result of an oversight.

We should deprecate such listeners. In the rare cases where a listener really wants to receive all events (like the audit), we'll introduce an explicit marker <event>*</event> to says so.