Show which item in a listing is selected, even if the listing isn't selected

This is foundation for actively highlighting the result of a jump-to
while typing. (Since your focus is on the ListingJumpElement, there
needs to be some way to see which item would be selected in the
GrouplikeListing. That's what this commit implements.)

This also looks pretty nifty when you press M to open the context menu,
if I do say so myself.

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