code: @UiChild(tagname="leftButton", limit=1)
      public void setLeftButton(JQMButton button) {
          if (left != null) remove(left);
          button.setPosOnBand(PosOnBand.LEFT);
          left = button;
          insert(left, 0);
      }
docs: Sets the left button to be the given button. Any existing button is removed.
  
  @param button - the button to set on the left slot