title: Issue Creating custom tfs query editor
body: <p>We are trying to implement workitem query editor in our application. So, do we have any readily available User control we can use for this?
  If not, wanted some insight into following:
  1. How are the operators relevant to different fields getting populated? EX: for System.TeamProject field only operator available in query editor is '=, even though it is a String field
  2. How is the value(allowed values) dropdown getting populated based on field and operator? Do we have any APIs for this?</p>
  
  <p>Thanks</p>