Fixed ActorGestureListener holding actor references forever.

These references can prevent GC!

gdx/src/com/badlogic/gdx/scenes/scene2d/utils/ActorGestureListener.java

@@ -114,22 +114,27 @@ public boolean handle (Event e) {
 			touchDown(event, tmpCoords.x, tmpCoords.y, event.getPointer(), event.getButton());
 			if (event.getTouchFocus()) event.getStage().addTouchFocus(this, event.getListenerActor(), event.getTarget(),
 				event.getPointer(), event.getButton());
+			actor = null;
 			return true;
 		case touchUp:
 			if (event.isTouchFocusCancel()) {
 				detector.reset();
+				touchDownTarget = null;
 				return false;
 			}
 			this.event = event;
 			actor = event.getListenerActor();
 			detector.touchUp(event.getStageX(), event.getStageY(), event.getPointer(), event.getButton());
 			actor.stageToLocalCoordinates(tmpCoords.set(event.getStageX(), event.getStageY()));
 			touchUp(event, tmpCoords.x, tmpCoords.y, event.getPointer(), event.getButton());
+			actor = null;
+			touchDownTarget = null;
 			return true;
 		case touchDragged:
 			this.event = event;
 			actor = event.getListenerActor();
 			detector.touchDragged(event.getStageX(), event.getStageY(), event.getPointer());
+			actor = null;
 			return true;
 		}
 		return false;
@@ -170,6 +175,7 @@ public GestureDetector getGestureDetector () {
 		return detector;
 	}
 
+	/** Returns null if there is no current touch down. */
 	public @Null Actor getTouchDownTarget () {
 		return touchDownTarget;
 	}