Add support for collapsing everything but top-level comments.

[adrxdr]

RES has something similar ("Hide all child comments" iirc) and it's pretty useful when you don't find the first couple of top-level comments too relevant / only want to follow some of the threads.

Also, I've seen there are a couple of pending PRs, do you still accept those?

Thanks!

[etcet]

That's a good idea, it could go next to 'expand all' but it's probably best if both 'expand all' and 'collapse children' should be moved above where the comments start if possible.

I will gladly take PRs but I'm not the most active person.