Polish CORS global configuration

This commit introduces the following changes:
 - configureCors(CorsConfigurer configurer) is renamed to
   addCorsMappings(CorsRegistry registry)
 - enableCors(String... pathPatterns) is renamed to
   addMapping(String pathPattern)
 - <cors /> element must have at least one <mapping /> child
   element in order to be consistent with XML based configuration
   and have more explicit configuration

Issues: SPR-12933, SPR-13046

spring-webmvc/src/test/resources/org/springframework/web/servlet/config/mvc-config-cors-minimal.xml

@@ -6,7 +6,9 @@
 		http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd">
 
 	<!-- <mvc:cors /> element before <mvc:annotation-driven /> one -->
-	<mvc:cors />
+	<mvc:cors>
+		<mvc:mapping path="/**" />
+	</mvc:cors>
 
 	<mvc:annotation-driven />