Add a Back button in the second step of the Embed form

d.o issue: https://drupal.org/node/2279269
We can put the empty function declaration directly inside the openDialog(), but it will not look very neat then. It will look like:
```html
Drupal.ckeditor.openDialog(editor, Drupal.url('entity-embed/dialog/entity_select/' + editor.config.drupal.format), existingValues={}, saveCallback=function(values){}, dialogSettings);
```
Also, values in not undefined. Its a parameter to the function.
IMO, changing this will make the function call look ugly. Let me know if you still want to change this.