Solution
Modify link variable like this (if application is admin):
$link = '../plugins/editors-xtd/myplugin/myplugin.layout.php?name='.$name;
... and delete button options (this means that file contents will be loaded via ajax inside modal)
Further more, in myplugin.layout.php we can add a little security check and we can import Joomla! framework library and defines so that we can make use of Joomla! framework in our file (things like language load for eg.) This is my actual header of file:
<?php
// No direct access
define('IS_AJAX', isset($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest');
if( ! IS_AJAX) die;
// Include J!Framework for later use
define( '_JEXEC', 1 );
define( 'JPATH_BASE', realpath(dirname(__FILE__).'/../../..'));
define( 'DS', DIRECTORY_SEPARATOR );
require_once ( JPATH_BASE.DS.'includes'.DS.'defines.php');
require_once ( JPATH_BASE.DS.'includes'.DS.'framework.php');
//more magic goes here...