XPTL - Add Filter option to selectable list

This adds a new PreValue option - "Enable Filtering" that allows users to filter the options list.  Makes working with large lists much simpler!

**Preview:**
![xpath-filter-sm](https://cloud.githubusercontent.com/assets/1396376/2869745/6cd337c8-d28a-11e3-8c15-33a45eafee96.gif)