title: How do I use a dropdown box as a column header filter in ng-grid / ui-grid?
body: <p>I am using ui-grid (angular's fast grid system).
  If I set 'enableFiltering: true', I get an text box under each column header that I can use to filter the grid, awesome.
  Now I need some or even all of these to be dropdown boxes instead of text boxes, how would I achieve this?</p>
  
  <p>e.g. The one column can only have the values 'OK', 'NOK' and 'ABB.'. So I would like to allow the user to filter that column based on only those 3 options.</p>