Allow checking whether there are notifiers registered

lib/private/notification/imanager.php

@@ -53,4 +53,10 @@ interface IManager extends IApp, INotifier {
 	 * @since 8.2.0
 	 */
 	public function createNotification();
+
+	/**
+	 * @return bool
+	 * @since 8.2.0
+	 */
+	public function hasNotifiers();
 }

lib/private/notification/manager.php

@@ -112,6 +112,14 @@ class Manager implements IManager {
 		return new Notification();
 	}
 
+	/**
+	 * @return bool
+	 * @since 8.2.0
+	 */
+	public function hasNotifiers() {
+		return !empty($this->notifiersClosures);
+	}
+
 	/**
 	 * @param INotification $notification
 	 * @return null