[UX] Manage display: use the 'dirty' form flag when field settings are changed, in order to indicate that the form needs to be saved.

One of the saddest UX failures of Drupal (and now Backdrop) is when someone updates field display settings four or five times with no effect. Usually this is because they click the "Update" button thinking their settings will update, but they don't. Not unless they also hit "Save". And the "Save" button is usually below the fold, so the poor unsuspecting user may not even know there is a save button.

Secret save

To solve this problem, we should make the update button actually save, too.

Related issue: https://github.com/backdrop/backdrop-issues/issues/573