Create a helper trait for Forms in ajax dialogs

Problem/Motivation

Settings Tray and the Workflow initiative are essentially solving the same common problems with using forms in AJAX dialogs ( #2785047: In Outside In mode, form validation messages should appear in the off-canvas tray, not the main page → and 📌 Use modals for creating, updating, and deleting workflows, with a new DialogFormTrait Needs review , respectively).

The problems were:

1. Showing validation messages on form submit error while remaining in the dialog
2. Refreshing the main page when needed
3. Redirect to a path on successful commit

Proposed resolution

Provide a common trait that can be used simply by including the trait and $this->buildFormDialog($form, $form_state, FALSE, '#drupal-modal'); (with the last 2 arguments being optional).

Remaining tasks

?

User interface changes

None.

API changes

API addition.

Data model changes

None.