Revert removal of world field.

This is needed. It shouldn't have been removed because it's nothing new.

worldedit-core/src/main/java/com/sk89q/worldedit/EditSession.java

@@ -205,6 +205,7 @@ public class EditSession extends AbstractDelegateExtent implements HasFaweQueue,
     }
 
     @SuppressWarnings("ProtectedField")
+    protected final World world;
     private String worldName;
     private FaweQueue queue;
     private boolean wrapped;