Present the user with a list of choices.
    allow him to select multiple items and return them in a list.
    if the user doesn't choose anything from the list, return the empty list.
    return None if he cancelled selection.

    :ref:`screenshots<multi_choice>`

    :param choices: a list of the choices to be displayed
    :param message: message to be displayed.
    :param title: window title
    :param default: default list of strings
    :rtype: None or list of strings

def multi_choice(choices=[], message='Pick as many items as you like.', default=None, title=''):
    """
    Present the user with a list of choices.
    allow him to select multiple items and return them in a list.
    if the user doesn't choose anything from the list, return the empty list.
    return None if he cancelled selection.

    :ref:`screenshots<multi_choice>`

    :param choices: a list of the choices to be displayed
    :param message: message to be displayed.
    :param title: window title
    :param default: default list of strings
    :rtype: None or list of strings
    """
    return backend_api.opendialog("multi_choice", dict(choices=choices, message=message, default=default, title=title))