Use Foundry's drag&drop functionality

Instead of the custom .addEventListener("dragstart", ...) and similar, use Foundry's drag&drop functionality:

getDefaultOptions({
  dragDrop: [
    {
      dragSelector: "dragelector",
      dropSelector: "dropselector",
    }
  ],
});

This may not matter too much, but it makes sure PF1 doesn't behave for some reason strangely in comparison to rest of Foundry.