Text area: WYSIWYG support

Follow up of #1947072: Allow linebreaks in table cells (text area).

A start to get WYSIWYG would be:

  $form['input_type'] = array(
    '#type' => 'radios',
    '#title' => t('Input type'),
    '#default_value' => isset($instance['widget']['settings']['input_type']) ? $instance['widget']['settings']['input_type'] : 'textfield',
    '#required' => TRUE,
    '#options' => array(
      'textfield' => t('textfield'),
      'textarea' => t('textarea'),
      'text_format' => t('text_format (wysiwyg)'),
    ),
    '#weight' => 120,
  );

Note 'text_format' => t('text_format (wysiwyg)'),. It would however need additional changes. Furthermore instead of storing only the value of the field it stores an array consisting of 'format' and 'value'. I then requires an is_array() condition throughout the code. Advantage is that the text_format can be set per cell.

Above patch can still be reviewed but only implements a simple textarea, not a WYSIWYG. After implementing that a new issue should be opened specifically asking for the feature request WYSIWYG.

Also probably this existing snippet should be made optional:

    if (module_exists('wysiwyg')) {
      $element['#wysiwyg'] = FALSE;
    }