extras/ExpandableListItem: Remove 'currentIndex' state when unhovering

The onExited signal gives us an opportunity to properly clean up
highlighting when mouse leaves the area, as otherwise positionChanged
is not emitted for out-of-bounds cursor (unlike the previous
containsMouseChanged signal).

Amends: 421219dc
See also: plasma-desktop!1027
See also: https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/562