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.
Thinking more about that... there's a slightly bigger tangle of issues here:

1) findNext() can be invoked with the search bar closed, and I think this code will crash in that case (`findBar` is null)

2) Before crashing, it will also compute an incorrect index in that case -- findNext() depends on the user's cursor position, which ca move around between invocations, so the next match it finds is not necessarily one increment away from the last match.  For now, this is safe since the search bar label is no longer visible; but [in the future](https://trello.com/c/TD4qOgxG/1084-find-replace-leave-search-bar-open-highlights-visible-when-editor-focused) this won't be true.  Should we future proof it now, or leave a note in the future story to remind ourselves to fix it later?

If we want to make it more robust now, one approach would be to always call `_findCurrentMatchIndex()`, but use the old `matchIndex` value as the starting search index.  That way, if the assumption above remains true, the search will never take more than 1 iteration -- but if the assumption is violated, it will still update the label correctly.

(A bigger cleanup could be to have findNext() actually use the stored resultSet when possible, instead of re-invoking the searchcursor code -- but I _don't_ think we should get into that now).