Having variables initialised from start `_isTransitioning` is better.
Would be better to add an eslint rule to check for undeclared variables use.
Reordered enter checks for `show` and `hide` by priority.

I'm using Tab.js with remove function and get an error "TypeError: container is undefined [more info]", with this check the error is fixed.

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.

in particular, no need to use originEvent, and preventDefault() only needed for touch events

these may also be the cause of weird behavior in Chrome/Surface, where scrolling vertically triggers slide advance

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

Broken after 787441d4.

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