Fix bugs with choose/unchoose

1. If the default was prevented, then performed, we were no longer
   checking to make sure it wasn't already added to the list of choices.
   This meant that if the implementor added it themselves it would be
   then added twice
2. The UI behavior performed once a choice was added was being
   grouped into the performDefault function, meaning if the implementor
   wanted to update the model themselves and prevented default, the UI
   would be out of sync. I've pulled that out of the performDefault
   function and now always do it since it has nothing to do with the data
   model