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 (merged)
See also: https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/562
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