Animation should stop on clear of parent

When an animation is removed, turns out it marks itself to stop. Unfortunately, the animations weren't getting added to the parent slot's collection, so the problem was that they never heard a remove at all!

I went down the path of adding `Shoes::Animation` directly to the slot contents like other elements, but I found myself including tons of junk just to appease the slots, things that didn't make sense on an animation like visibility, dimensioning, etc.

After the third not-really-necessary-but-gotta-add-it thing, I decided to change direction and add a hook in the slot class to listen for `clear` calls without being in a slot's contents. This was much, much simpler and get us solved on #935 handily.