This is the first step for listener level handler properties.
To allow a handler inside a polymorphic listener to be non-polymorphic,
the @Polymorphic annotation now has a boolean value that defaults to
true. In that case, it can be explicitly set to false to override the
listener-level default.
The new @Priority annotation serves the exact same purpose as
@Event(priority = ...), but should be easier to read in complex handler
declarations. It has to be used in conjunction with the @Event
annotation, not instead of it.