Few improvements to CollapsingState

[4JX]

I've made two patches to CollapsingState to add some new functionality:

• Changing the side the collapser is drawn at
• Custom icon support

However, I'd like to get a check on that second feature (4JX@73d520a), since it does some possibly breaking changes (Fn should still suffice for 100% of icon functions, but just in case) before opening a PR.

I also wanted to implement a "no icon" mode for CollapsingState, where it would not ever call the draw icon function. The issue is that I would have to introduce an Option inside the HeaderResponse.toggle_button_response, making the usage of that response slightly unergonomic. Do you think it'd be a problem to do it this way?

[emilk]

Breaking changes are fine (even though we of course try to minimize them). I think this will be easier to discuss if you open a draft PR!