Fix field widget settings form to update after handle change

Problem/Motivation

Although the module is functional, it would be nice to figure out the form state issue when changing a handler so that the form can be saved once instead of having to go back into it. This is most-likely due to some incompatibilities with the form process, ajax etc... from code borrowed from Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem

Steps to reproduce

1. Edit the the field widget settings
2. Change the Reference handler from Default to Views
3. The Handler settings form does not update.

Proposed resolution

To be determined.

Remaining tasks

1. Write a Functional test (as this occurs whether JavaScript is enabled or not).
2. Figure out the complexities behind Entity Form Display and Entity Reference Handler Settings plugins, and fix the issue.