added interface for storage classes

lib/model/repositories/interfaces/task_repository.dart

@@ -0,0 +1,15 @@
+import '../../task.dart';
+
+abstract class TaskRepository {
+  Future<List<Task>> loadTasks();
+
+  Future<Iterable<String>> loadTaskOrder();
+
+  Future<void> saveTaskOrder(List<String> taskOrder);
+
+  Future<void> saveTask(Task task);
+
+  Future<void> saveTasks(List<Task> tasks);
+
+  Future<void> deleteTask(Task task);
+}