Fix overlapping transitions with parent x-shows

[calebporzio]

Before this PR, given the following code:

<div x-data="{show: true}">
  <button @click="show = ! show">Toggle</button>

  <div x-show="show">
    <div x-show.transition="show">
      // Contents
    </div>
  </div>
</div>

If you press the "toggle" button, then press it AGAIN while the "out" transition is still playing, instead of transitioning back in mid-way, the contents would immediately disappear.

This only happens when an x-transition is wrapped by an x-show without a transition.

This PR fixes this case, but rejecting the entire show directive stack at the component level once any transition within it has been cancelled.