migrate Callback to Interface

- closes #5717

forge-gui/src/main/java/forge/util/Callback.java

@@ -1,5 +1,6 @@
 package forge.util;
 
-public abstract class Callback<T> {
-    public abstract void run(T result);
+@FunctionalInterface
+public interface Callback<T> {
+    void run(T result);
 }

forge-gui/src/main/java/forge/util/WaitCallback.java

@@ -2,7 +2,7 @@ package forge.util;
 
 import forge.gui.FThreads;
 
-public abstract class WaitCallback<T> extends Callback<T> implements Runnable {
+public abstract class WaitCallback<T> implements Callback<T>, Runnable {
     public class Lock {
     }