void ApplicationState::addDrawableObject(DrawableObject& drawable)
{
  drawableObjects.push_back(&drawable);

  if (drawable.isClickable())
    addClickableObject(drawable, drawable.getLayer());
}
void  ApplicationState::addClickableObject(DrawableObject& clickable, int z)
{
  if (!clickable.isClickable())
    return;
  clickableObjects[z].insert(clickable);
}