Refactor API notification usage

- Remove "mapFields()" from BaseModel
- Add new Notification API entity (including collection)
- Add new NotificationFactory method "getApiList()"

src/Collection/Api/Notifications.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace Friendica\Collection\Api;
+
+use Friendica\BaseCollection;
+use Friendica\Object\Api\Friendica\Notification;
+
+class Notifications extends BaseCollection
+{
+	/**
+	 * @return Notification
+	 */
+	public function current()
+	{
+		return parent::current();
+	}
+}