util: prepare to update to JDOM 2.0 (IDEA-192733)

Some methods in Content's inheritors use covariant return types in JDOM 2.0 so we need to update our implementations.

platform/util/src/org/jdom/ImmutableCDATA.java

@@ -22,7 +22,7 @@ public class ImmutableCDATA extends CDATA {
 
   @SuppressWarnings("MethodDoesntCallSuperMethod")
   @Override
-  public Text clone() {
+  public CDATA clone() {
     CDATA text = new CDATA();
     text.value = value;
     return text;
@@ -35,7 +35,7 @@ public class ImmutableCDATA extends CDATA {
 
   //////////////////////////////////////////////////////////////////////////
   @Override
-  public Text setText(String str) {
+  public CDATA setText(String str) {
     throw ImmutableElement.immutableError(this);
   }
 
@@ -50,12 +50,12 @@ public class ImmutableCDATA extends CDATA {
   }
 
   @Override
-  public Content detach() {
+  public CDATA detach() {
     throw ImmutableElement.immutableError(this);
   }
 
   @Override
-  protected Content setParent(Parent parent) {
+  protected CDATA setParent(Parent parent) {
     throw ImmutableElement.immutableError(this);
     //return null; // to be able to add this to the other element
   }

platform/util/src/org/jdom/ImmutableElement.java

@@ -304,7 +304,7 @@ class ImmutableElement extends Element {
 
   //////////////////////////////////////////////////////////////////////
   @Override
-  public Content detach() {
+  public Element detach() {
     throw immutableError(this);
   }
   @Override

platform/util/src/org/jdom/ImmutableText.java

@@ -50,12 +50,12 @@ public class ImmutableText extends Text {
   }
 
   @Override
-  public Content detach() {
+  public Text detach() {
     throw ImmutableElement.immutableError(this);
   }
 
   @Override
-  protected Content setParent(Parent parent) {
+  protected Text setParent(Parent parent) {
     throw ImmutableElement.immutableError(this);
     //return null; // to be able to add this to the other element
   }