Fix two kickoff key hanlding issues
If there's only one item, we still need to to process the newly added
enter key handling that has moved here.
increment/decrement current index will just no-op. We still need to
guard count > 0 as "currentItem" is used.
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applets/kickoff: Accept key event after handling action triggering
If the listview consumes the key event, no-one else should also perform
an action. This is important as some questionable key forwarding logic
in Kickoff can apparently end up with keys processed by the ListView
twice if they're not consumed.
BUG: 492842