Add Display plugin options

d.o issue: https://drupal.org/node/2283691

When this issue goes through, following follow-up issues will be needed:

* Remove redundant 'view-mode' field from the form.
* Use rebuildEmbedForm() in submitForm() when form['state'] is select.
Maybe we can just show the list of view-modes and remove this field.
But I think that should be done as part a follow-up issue after we decide what to do with the view-mode field.