Adding optionsFiltered event

This adds an event that is triggered each time the list of items is modified by the search input. The current search term is passed as part of the event.

The purpose of this is to allow post-processing of the items after each search - in my particular case it's because I want to unhide the currently selected item when appropriate (i.e. if it matches the search term, or if there is no longer a search filter in effect).