code: function( index ) {
          var id = "impress-toolbar-group-" + index;
          if ( !groups[ index ] ) {
              groups[ index ] = document.createElement( "span" );
              groups[ index ].id = id;
              var nextIndex = getNextGroupIndex( index );
              if ( nextIndex === undefined ) {
                  toolbar.appendChild( groups[ index ] );
              } else {
                  toolbar.insertBefore( groups[ index ], groups[ nextIndex ] );
              }
          }
          return groups[ index ];
      }
docs: Get the span element that is a child of toolbar, identified by index.
  
  If span element doesn't exist yet, it is created.
  
  Note: Because of Run-to-completion, this is not a race condition.
  https://developer.mozilla.org/en/docs/Web/JavaScript/EventLoop#Run-to-completion
  
  :param: index   Method will return the element <span id="impress-toolbar-group-{index}">