code: public SwipeViewGroup addBackground(View background, SwipeDirection direction){
          if(mBackgroundMap.get(direction) != null) removeView(mBackgroundMap.get(direction));
  
          background.setVisibility(View.INVISIBLE);
          mBackgroundMap.put(direction, background);
          addView(background);
          return this;
      }
docs: Add a View to the background of the Layout. The background should have the same height
  as the contentView
  
  @param background The View to be added to the Layout
  @param direction The key to be used to find it again
  @return A reference to the a layout so commands can be chained