Add DefaultEvent flag to fix notification for relogin/restart

Summary:
This flag is necessary is no notifyrc for the given appName exists.
Otherwise the notification is silently discarded.

Test Plan:
Added call to onRestartRequired somewhere more easily reachable,
without this patch no notification appears. With this patch,
a notification appears.

Reviewers: #plasma, lukas, jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7825