kcms/notifications: Allow to focus an specific event on launch

When called with the argument --event-id, do not only go the specific app settings, but also focus into the specified event.

The argument parsing and scaffolding was already there, but the final bit of functionality was lost in the port to the QML list of events.

Example: kcmshell6 --highlight kcm_notifications --args "--notifyrc=plasma_workspace --event-id=fatalerror"