[Notifications] Group only same origin and show it in heading

This makes it group notifications of different "origins" not group together,
e.g. FooApp notifications from phone A won't be grouped together with FooApp notifications from phone B.
Also, email notifications from different accounts wouldn't be grouped together.
Finally, show the origin in the grouped header.

Differential Revision: https://phabricator.kde.org/D23583