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.

ui/src/main/java/dev/quoije/umd/ui/RcoRenderer.java

@@ -112,7 +112,12 @@ public final class RcoRenderer implements Display {
     }
 
     private void draw(GraphicsContext g, IDisplay display) {
-        Image image = decode(display.getImage());
+        Object source = display.getImage();
+        if (display == focus && display.getFocusImage() != null) {
+            source = display.getFocusImage();
+        }
+
+        Image image = decode(source);
         if (image == null) {
             return;
         }