[NDS] Improve VisualElement class. Add GameObject. Add touch action.

- Improve the VisualElement class so it's possible to load multiple
sprites at the same time. And when moving, each one will be shown in the
correctly position (so far, haha);
- Add game object;
- Add touch action (this was the mainly goal for this commit, but the
game object was needed to test the action, then, the visual element
class wasn't working with multiple sprites because a misconcept of
myself).