API for handling selected completion item actually works; added test

IDEA-CR-31904

java/java-tests/testData/codeInsight/completion/normal/Simple_afterCompletionService.java

@@ -0,0 +1,20 @@
+class A extends Base{
+  int _field;
+  void _method(){}
+
+  void foo(){
+    int _local1;
+    int _local2;
+    _field
+    int _local3;
+  }
+}
+
+class Base{
+  void _baseMethod(){}
+  int _baseField;
+
+  private void privateMethod(){}
+  private int privateField;
+}
+