When polymorphism or a priority is defined at listener-level and then
identically defined for a specific handler, a warning is issued.
This does not affect redefinitions of default values.
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.