let the world know our matches have changed when they, well, change. always.

(neat when optimizations reveal previously when we were always setting the match lists even when we got empty lists, this was masking the fact that we were never updating the matches when the search context was reset ... so there was always a period of time when there were stale matches pointing off into outerspace in the UI. and of course, if no engines returned anything .. booM! but once we were not responding to empty match lists this problem became very visible.)