Aims to make minimal changes to `DrawableSlider` itself. I'm not super
happy about the slider ball being moved above the head circle, but it
*is* what people are used to so no one except for me is going to
complain.
Supersedes and closes https://github.com/ppy/osu/pull/14561.
The `OnComplete` event was never being run due to the transform playing
out longer than the spinner's lifetime. I've matched the durations, but
also moved the `Stop()` call to what I deem a safer place to run it (I
did notice that without this it would still potentially never fire).
Note that this is more noticeable in the editor because of lifetime
extension. In gameplay, the returning of a spinner to the pool will
clean things up (but in the editor that can take longer, depending on
timeline zoom level).
Another thing worth mentioning is that the fade doesn't actually work.
This is due to https://github.com/ppy/osu-framework/pull/4212.
Closes#12119.