[SPR-8386][SPR-8387] Redesign of DelegatingSmartContextLoader and the SmartContextLoader SPI:

- Removed generatesDefaults() and supports() from the SmartContextLoader SPI and modified the DelegatingSmartContextLoader algorithm accordingly.
- DelegatingSmartContextLoader no longer operates on a list of candidate loaders but rather on explicit instances of GenericXmlContextLoader and AnnotationConfigContextLoader.
- Updated Javadoc regarding DelegatingSmartContextLoader as the new default loader.
- Updated and polished Javadoc regarding changes in 3.1.
- Now enforcing @ContextConfiguration's restriction on declaring locations or classes but not both.

org.springframework.test/src/test/resources/org/springframework/test/context/support/DelegatingSmartContextLoaderTests$ImproperDuplicateDefaultXmlAndConfigClassTestCase-context.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xmlns:c="http://www.springframework.org/schema/c"
+	xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.1.xsd">
+
+	<!-- intentionally empty: we just need the file to be present to fail the test -->
+
+</beans>