code: public SwipeActionAdapter addBackground(SwipeDirection key, int resId){
          if(SwipeDirection.getAllDirections().contains(key)) mBackgroundResIds.put(key,resId);
          return this;
      }
docs: Add a background image for a certain callback. The key for the background must be one of the
  directions from the SwipeDirections class.
  
  @param key the identifier of the callback for which this resource should be shown
  @param resId the resource Id of the background to add
  @return A reference to the current instance so that commands can be chained