Make alternative text required in CKeditor image dialog.

readme.md

This makes the alt text required in ckeditor and also adds Drupal's required span.

gistfile1.js

CKEDITOR.on('dialogDefinition', function(ev) {
  var dialogName = ev.data.name;
  var dialogDefinition = ev.data.definition;
  var dialog = ev.data.definition.dialog;

  switch (dialogName) {
    case 'image': //Image Properties dialog

      // Require alt text.
      var altField = infoTab.get('txtAlt');
      altField.label = 'Alternative text <span class="form-required" title="This field is required.">*</span>';
      altField.validate = function() {
        var alt = dialog.getContentElement('info', 'txtAlt');
        if (alt.getValue() == '') {
          return "Alternative text is required.";
        }
      }
      break;
    }
});

for image2, just change 'image' to 'image2' and 'txtAlt' to 'alt'