Fix .collapsed markup for accordions

[simonihmig]

Extracted the bug fix from #1561

[simonihmig]

@jelhan do you want to / have the time to review this? I think this should not be very controversial, just a lot of noise in the tests diff. So if you don't have the time, then I would like to move this forward quickly nevertheless, also as it blocks #1561...

[jelhan]

Looks good to me.

I'm just a little bit confused that I haven't found .expanded class in BS3 nor BS4 docs. Are you sure that class is needed? Or is that maybe another big?

[simonihmig]

I'm just a little bit confused that I haven't found .expanded class in BS3 nor BS4 docs. Are you sure that class is needed? Or is that maybe another big?

Hm, I wasn't aware tbh. Probably something along the lines of #1261, where some classes were added for convenience (e.g. styling an expanded item specifically, without putting the styles into all items, and overriding them for .collapsed). But I think we can remove that in v5 if we want. All supported browsers will then also support :not()...