This replaces our *custom* overlays with standard `<dialog>` DOM elements, see https://developer.mozilla.org/en-US/docs/Web/HTML/Element/dialog, thus simplifying the related CSS, HTML, and JavaScript code.
With these changes, some of the functionality of the `OverlayManager` class is now handled natively (e.g. `Esc` to close the dialog). However, since we still need to be able to prevent dialogs from overlaying one another, it still makes sense to keep this functionality (as far as I'm concerned).