Show Undo in prompt if last action is undoable

Clean up unnecessary items from Game menu

forge-game/src/main/java/forge/game/zone/MagicStack.java

@@ -229,6 +229,17 @@ public class MagicStack /* extends MyObservable */ implements Iterable<SpellAbil
         return this.bResolving;
     }
 
+    /**
+     * <p>
+     * undo.
+     * </p>
+     * 
+     * @return a boolean.
+     */
+    public final boolean canUndo() {
+        return !undoStack.isEmpty();
+    }
+
     /**
      * <p>
      * undo.