code: @Nonnull
    public static BootstrapButtonGroup createButtonAsDropDownMenu (@Nonnull final BootstrapButton aButton,
                                                                   @Nonnull final Consumer <? super BootstrapDropdownMenu> aMenuItemProvider)
    {
      final BootstrapButtonGroup aBG = new BootstrapButtonGroup ();
      BootstrapDropdown.makeDropdownToggle (aButton);
      aBG.addChild (aButton);
  
      final BootstrapDropdownMenu aMenu = aBG.addDropDownMenu ();
      aMenuItemProvider.accept (aMenu);
      return aBG;
    }
docs: Create a new button group with a default button and an additional drop down
  part.
  
  @param aButton
  The button to be added. May not be <code>null</code>.
  @param aMenuItemProvider
  The drop down menu item provider.
  @return The created button group. Never <code>null</code>.