Merge BeanWrapperImpl and DirectFieldAccessor

`BeanWrapperImpl` and `DirectFieldAccessor` are two
`ConfigurablePropertyAccessor` implementations with different features
set.

This commit harmonizes the two implementations to use a common base class
that delegates the actual property handling to the sub-classes:

* `BeanWrapperImpl`:  `PropertyDescriptor` and introspection utilities
* `DirectFieldAccessor`: reflection on `java.lang.Field`

Issues: SPR-12206 - SPR-12805

spring-web/src/test/java/org/springframework/web/bind/support/WebRequestDataBinderTests.java

@@ -345,11 +345,11 @@ public class WebRequestDataBinderTests {
 
 	static class TestBeanWithConcreteSpouse extends TestBean {
 		public void setConcreteSpouse(TestBean spouse) {
-			this.spouses = new ITestBean[] {spouse};
+			setSpouse(spouse);
 		}
 
 		public TestBean getConcreteSpouse() {
-			return (spouses != null ? (TestBean) spouses[0] : null);
+			return (TestBean) getSpouse();
 		}
 	}