Add explicit return types to IUserNotificationEvent::getTitle() and :…

…:getLink()

These are required since #5157, because the actual event classes implement both
this interface and the ITitledObject/ILinkableObject interfaces, requiring both
interfaces to be compatible.

This breaks the `null` return value for `getLink()`, but implementing classes
can just implement the compatibility link to NotificationListPage themselves.

wcfsetup/install/files/lib/system/user/notification/event/IUserNotificationEvent.class.php

@@ -22,10 +22,8 @@ interface IUserNotificationEvent extends IDatabaseObjectProcessor
 {
     /**
      * Returns a short title used for the notification overlay, e.g. "New follower".
-     *
-     * @return  string
      */
-    public function getTitle();
+    public function getTitle(): string;
 
     /**
      * Returns the notification event message, e.g. "dtdesign is now following you".
@@ -36,15 +34,8 @@ public function getMessage();
 
     /**
      * Returns object link.
-     *
-     * The returned value can be `null` however, this should be avoided as much as possible.
-     * If the returned value is `null`, the NotificationListPage will be used a link, on any
-     * place, where the link is required (e.g. in the NotificationConfirmAction). After the
-     * confirmation of the notification, the notification is not clickable anymore.
-     *
-     * @return  ?string
      */
-    public function getLink();
+    public function getLink(): string;
 
     /**
      * Returns the full title for this notification, e.g. for use with e-mails.