Add new notifications at the top of the list

Summary:
This will show a list with the most recent notifications at the top, in the
plasmoid.

This is more consistent with the KDE notification plasmoid that stack them like
this too.

Reviewers: apol, KDE Connect

Reviewed By: apol, KDE Connect

Subscribers: kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D13540