Summary: This allows one to subscribe to notifications from notification server.
Test Plan:
tested using very simple QML
```
import QtQuick 2.0
import org.kde.notificationmanager 1.1 as Notifications
...
Notifications.NotificationWatchedModel {
id: model
}
```
Reviewers: #plasma, broulik, davidedmundson
Reviewed By: #plasma, broulik
Subscribers: nicolasfella, plasma-devel
Tags: #plasma
Differential Revision: https://phabricator.kde.org/D28509
Bhushan Shah