Dynamically set/remove `aria-modal="true"` when a modal is shown/hidden

Firefox currently seems extremely fickle - with `pan-y` if fires pointercancel as soon as a touch strays even a pixel or so vertically.
While `touch-action: pan-y` would be ideal (allowing users to scroll the page even when their finger started the scroll on the carousel), this prevents a swipe that isn't perfectly/only horizontal to be recognised by Firefox.

- my fault, my original advice of using `touch-action: pan-x` is exactly the value we *don't* want to have the browser handle...

adds the ability to assign data-interval to an individual carousel-item