Fire onChapter on chapter boundaries; M returns to menu via onMenu

The menu state machine advances via onChapter events when a video crosses a
chapter marker, which we never emitted - so the menu never opened naturally.
Track chapter boundaries from the playlist markers and fire onChapter. Also
change M to invoke the onMenu dispatcher (returns to the menu video) rather
than drawing menu items directly.

core/src/main/java/dev/quoije/umd/rco/vsmx/objects/MoviePlayer.java

@@ -80,6 +80,8 @@ public class MoviePlayer extends BaseNativeObject {
 
         if (!previousMenuMode && this.menuMode) {
             VsmxBaseObject callback = controller.getPropertyValue("onMenu");
+            System.err.println("[menu] onMenu fires, callback=" + callback
+                    + " (" + (callback == null ? "null" : callback.getClass().getSimpleName()) + ")");
             if (callback instanceof VsmxFunction function) {
                 interpreter.interpretFunction(function, null, new VsmxBaseObject[0]);
             }