Add presentation layer for mailbox, mail, message

model/lib/mail/presentation_thread.dart

@@ -0,0 +1,17 @@
+
+import 'package:equatable/equatable.dart';
+
+class PresentationThread with EquatableMixin {
+
+  final String id;
+  final String? message;
+
+  PresentationThread(this.id, {this.message});
+
+  factory PresentationThread.createThreadEmpty() {
+    return PresentationThread('empty');
+  }
+
+  @override
+  List<Object?> get props => [id, message];
+}