Fix #5445 - Limit Accordion aunt/sibling selections

Accordion has been greedy, picking up any nested elements similar to itself (nested Accordion, Tabs, etc.). This should limit the accordion's `active` state change to only itself, no nested content.

(Sorry for sloppy commits. If you see anything in there before the revert you want, i'll make a PR for it.)
Original selectors led to this result. Maybe "cousins" is a more appropriate name? :)