No event.relatedTarget for Collapse

[aguegu]

I have a form in a moral to do the edit job. And then the form become too big that it does not fit in the modal. Then I realise that collapse could be a good alternative for the modal.

And then I noticed that collapse event does not have the .relatedTarget property as modal events. I have to get some data from the relatedTarget to render the form.

[cvrebert]

Approximate duplicate of #15393.