Replace t() with $this->t() in all plugins

Problem/Motivation

When attempting to unit test a widget which extends the StringTextfieldWidget the traditional method of mocking the string translation service fails and an error is produced Call to undefined function Drupal\Core\Field\Plugin\Field\FieldWidget\t().

Proposed resolution

PluginBase is using StringTranslationTrait but many of the child plugins are not utilizing it properly. Use $this->t() instead of t() for string translation calls for all classes extending PluginBase.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet