Render focused button's focus image in the menu

Buttons with a focus image now swap to it while selected, giving visible
feedback when navigating with the arrow keys.

core/src/main/java/dev/quoije/umd/rco/IDisplay.java

@@ -8,6 +8,11 @@ public interface IDisplay {
 
     Object getAnimImage();
 
+    /** The image shown while this object has input focus, or {@code null}. */
+    default Object getFocusImage() {
+        return null;
+    }
+
     int getX();
 
     int getY();

core/src/main/java/dev/quoije/umd/rco/object/ButtonObject.java

@@ -29,6 +29,11 @@ public class ButtonObject extends BasePositionObject {
         return image.getImage();
     }
 
+    @Override
+    public Object getFocusImage() {
+        return focus.getImage();
+    }
+
     @Override
     public void setFocus() {
         trigger(onFocusIn);