Simplify notification handling
Summary:
Use QPointer for KNotification
Use ready signal for signalling updates
BUG: 400010
Test Plan: Spawned some notifications
Reviewers: KDE Connect, broulik, albertvaka
Reviewed By: KDE Connect, albertvaka
Subscribers: albertvaka, kdeconnect
Tags: KDE Connect
Differential Revision: https://phabricator.kde.org/D18354