Show match index in place of match count

This is the implementation of https://trello.com/c/0GXqt5GF/1089-s-find-next-indicate-current-match-index.
Might want to add a note here that `findNext()`'s call to `_showMatchIndex()` is what upates this label in the other case -- and that any call to `updateResultSet()` is _guaranteed_ to be followed by a call to `findNext()` (unless there's no query, in which case we've already cleared the find count label above).