Modern Label printing (#724)

* Basic widget

* Redirect label printing action

* Refactor label printing code

* Construct form elements

* Refactor to allow re-use of forms

* Basic rendering of label printing form

* Remove dead code

* Pass custom handler through to form context

* Refactoring API forms:

- Allow custom pk field name
- Add callback when values change

* linting

* Dynamically rebuild form

* Handle nested fields

* Handle label printing status

* Run dart format

* Update release notes

* Remove unused var

* Enable close icon

* Handle initial plugin default value

* Store default values:

- Selected template (per label type)
- Selected printing plugin

* Dart format

* Fix dart linting

* use setter

* Just use a public field

lib/widget/snacks.dart

@@ -64,6 +64,7 @@ void showSnackIcon(
         },
       ),
       backgroundColor: backgroundColor,
+      showCloseIcon: true,
       action: onAction == null
           ? null
           : SnackBarAction(