New function to handle combobox config items added to ACP

Putting extension configuration beside similar phpbb core configuration is the best behavior in many cases when the extension do a little change to phpBB work, 'core.acp_board_config_edit_add' event do this good, but extension writer can't add a combobox config ('type' => 'select') because it requires a core function or method, so I suggest simple function handle this.