Restore AbstractPropertyAccessor

Commit 3d86f15 added a lot of new feature in AbstractPropertyAccessor
shared by both DirectFieldAccessor and BeanWrapperImpl. This makes this
abstract class harder to implement for others having simpler use cases.

It turns that Spring Data has such use case; this commit split these new
features in a dedicated new base class, leaving AbstractPropertyAccessor
untouched.

Issue: SPR-12805

spring-beans/src/main/java/org/springframework/beans/BeanWrapperImpl.java

@@ -60,7 +60,7 @@ import org.springframework.util.Assert;
  * @see BeanWrapper
  * @see PropertyEditorRegistrySupport
  */
-public class BeanWrapperImpl extends AbstractPropertyAccessor implements BeanWrapper {
+public class BeanWrapperImpl extends AbstractNestablePropertyAccessor implements BeanWrapper {
 
 	/**
 	 * Cached introspections results for this object, to prevent encountering

spring-beans/src/main/java/org/springframework/beans/DirectFieldAccessor.java

@@ -44,7 +44,7 @@ import org.springframework.util.ReflectionUtils;
  * @see org.springframework.validation.DirectFieldBindingResult
  * @see org.springframework.validation.DataBinder#initDirectFieldAccess()
  */
-public class DirectFieldAccessor extends AbstractPropertyAccessor {
+public class DirectFieldAccessor extends AbstractNestablePropertyAccessor {
 
 	private final Map<String, FieldPropertyHandler> fieldMap = new HashMap<String, FieldPropertyHandler>();