fix(notification-container): Fix wrong ngFor trackby implementation

- Fix wrong ngFor trackby implementation by replacing the direct value access with a class method

Closes #12.

src/components/notifier-container.component.html

@@ -1,5 +1,5 @@
 <ul>
-	<li class="x-notifier__container-list" *ngFor="let notification of notifications; trackBy: notification?.id">
+	<li class="x-notifier__container-list" *ngFor="let notification of notifications; trackBy: identifyNotification;">
 		<x-notifier-notification
 			[notification]="notification"
 			(ready)="onNotificationReady( $event )"

src/components/notifier-container.component.ts

@@ -96,6 +96,17 @@ export class NotifierContainerComponent implements OnDestroy, OnInit {
 		}
 	}
 
+	/**
+	 * Notification identifier, used as the ngFor trackby function
+	 *
+	 * @param   {number}               index        Index
+	 * @param   {NotifierNotification} notification Notifier notification
+	 * @returns {string}                            Notification ID as the unique identnfier
+	 */
+	public identifyNotification( index: number, notification: NotifierNotification ): string {
+		return notification.id;
+	}
+
 	/**
 	 * Event handler, handles clicks on notification dismiss buttons
 	 *