This patch puts the Discontinuity (Euler) Filter button in the Dope Sheet->Key menu, so it's not only available from the Graph Editor->Key menu. On request of @Pablo Fournier (pablico), see T95386.

This required changing a poll function which is used by a bunch of other operators, which seemed scary at first, but my thinking is that if an operator can execute in the Graph Editor, then it should also be able to execute in the Dope Sheet. I think the only reason this wouldn't be true is if we were storing animation data in the UI itself, which of course we don't. So I hope this is okay.

If this turns out to be more complicated than it seemed at first glance, then my C copy pasting skills probably won't cut it. ;)