added activity events for plugins (#2516)

src/Event/ActivityUpdatePostEvent.php

@@ -0,0 +1,17 @@
+<?php
+
+/*
+ * This file is part of the Kimai time-tracking app.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace App\Event;
+
+/**
+ * Triggered for activity instances, which were updated.
+ */
+final class ActivityUpdatePostEvent extends AbstractActivityEvent
+{
+}