API endpoints to delete customer/project/activity (#5181)

* added service methods with events to delete customer, project, activity
* added API endpoints to delete customer, project, activity
* added tests for new API endpoints

src/Event/ActivityDeleteEvent.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 right before a activity will be deleted.
+ */
+final class ActivityDeleteEvent extends AbstractActivityEvent
+{
+}