FacesConfigValidator wrongly checks for NavigationHandler implementation

When validating faces-config, a custom navigation-handler class gets reported as "does not implement interface javax.faces.application.NavigationHandler". NavigationHandler is not an interface but an abstract class.

FacesConfigValidator, when creating check for navigation-handler in createChecks(), should call new JSFCheckClass with 'javax.faces.application.NavigationHandler' in extends-type parameter.