Allow iFrame displays created trough the UI to have 100% width

Problem/Motivation

The current validation of the iFrame config values does not allow to input a 100% width for the iFrame display:

  /**
   * {@inheritdoc}
   */
  public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
    if ($form_state->getValue('width') <= 0) {
      $form_state->setError($form['width'], $this->t('Width must be greater than 0.'));
    }
    if ($form_state->getValue('height') <= 0) {
      $form_state->setError($form['height'], $this->t('Height must be greater than 0.'));
    }
  }

Modules that provide default browser do set this to 100%, for instance the file_browser's one:

display_configuration:
  widget_ids:
    735d146c-a4b2-4327-a057-d109e0905e05: 'Upload files'
    a4ad947c-9669-497c-9988-24351955a02f: 'Files listing'
  width: 100%
  height: 768
  link_text: 'Select files'
  auto_open: true

This implies that a site builder cannot modify this browser trough the UI using the configuration form.

Proposed resolution

Improve the validation of this display plugin to allow the width of the iFrame display to be used either in px or in %.